feat(web): add before/after image comparison slider component

Draggable split-view slider with clip-path reveals original vs processed
image. Shows file size badges with savings percentage. Supports pointer
events for both mouse and touch. Integrated into tool-page to replace
dropzone after processing completes. Updated file-store with
originalBlobUrl for comparison. Enhanced dropzone with multi-file count
badge and file list display.

apps/web/src/components/common/before-after-slider.tsx

@@ -0,0 +1,178 @@
+import { useRef, useState, useCallback, useEffect, type PointerEvent } from "react";
+
+interface BeforeAfterSliderProps {
+  /** URL or data URL of original image. */
+  beforeSrc: string;
+  /** URL or data URL of processed image. */
+  afterSrc: string;
+  /** Original file size in bytes. */
+  beforeSize?: number;
+  /** Processed file size in bytes. */
+  afterSize?: number;
+}
+
+function formatSize(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  return `${(bytes / (1024 * 1024)).toFixed(2)} MB`;
+}
+
+/**
+ * Before/After image comparison slider.
+ *
+ * Shows two images overlapping with a draggable vertical divider.
+ * The "before" image is on the left, "after" on the right.
+ * Supports mouse and touch interaction via pointer events.
+ */
+export function BeforeAfterSlider({
+  beforeSrc,
+  afterSrc,
+  beforeSize,
+  afterSize,
+}: BeforeAfterSliderProps) {
+  const containerRef = useRef<HTMLDivElement>(null);
+  const [position, setPosition] = useState(50); // percentage 0-100
+  const [isDragging, setIsDragging] = useState(false);
+
+  const updatePosition = useCallback(
+    (clientX: number) => {
+      const container = containerRef.current;
+      if (!container) return;
+      const rect = container.getBoundingClientRect();
+      const x = clientX - rect.left;
+      const pct = Math.max(0, Math.min(100, (x / rect.width) * 100));
+      setPosition(pct);
+    },
+    [],
+  );
+
+  const handlePointerDown = useCallback(
+    (e: PointerEvent) => {
+      e.preventDefault();
+      setIsDragging(true);
+      (e.target as HTMLElement).setPointerCapture(e.pointerId);
+      updatePosition(e.clientX);
+    },
+    [updatePosition],
+  );
+
+  const handlePointerMove = useCallback(
+    (e: PointerEvent) => {
+      if (!isDragging) return;
+      updatePosition(e.clientX);
+    },
+    [isDragging, updatePosition],
+  );
+
+  const handlePointerUp = useCallback(() => {
+    setIsDragging(false);
+  }, []);
+
+  // Prevent default drag behavior on images
+  useEffect(() => {
+    const container = containerRef.current;
+    if (!container) return;
+    const preventDrag = (e: Event) => e.preventDefault();
+    container.addEventListener("dragstart", preventDrag);
+    return () => container.removeEventListener("dragstart", preventDrag);
+  }, []);
+
+  const savingsPercent =
+    beforeSize && afterSize && beforeSize > 0
+      ? ((1 - afterSize / beforeSize) * 100).toFixed(1)
+      : null;
+
+  return (
+    <div className="flex flex-col items-center gap-3 w-full max-w-2xl mx-auto">
+      {/* Slider container */}
+      <div
+        ref={containerRef}
+        className="relative w-full overflow-hidden rounded-lg border border-border select-none touch-none"
+        style={{ cursor: isDragging ? "ew-resize" : "default" }}
+        onPointerDown={handlePointerDown}
+        onPointerMove={handlePointerMove}
+        onPointerUp={handlePointerUp}
+        onPointerCancel={handlePointerUp}
+      >
+        {/* Before image (full width, bottom layer) */}
+        <img
+          src={beforeSrc}
+          alt="Original"
+          className="block w-full h-auto"
+          draggable={false}
+        />
+
+        {/* After image (clipped, top layer) */}
+        <img
+          src={afterSrc}
+          alt="Processed"
+          className="absolute inset-0 w-full h-full object-contain"
+          draggable={false}
+          style={{
+            clipPath: `inset(0 0 0 ${position}%)`,
+          }}
+        />
+
+        {/* Divider line */}
+        <div
+          className="absolute top-0 bottom-0 w-0.5 bg-white/80 pointer-events-none"
+          style={{ left: `${position}%`, transform: "translateX(-50%)" }}
+        >
+          {/* Handle grip */}
+          <div className="absolute top-1/2 left-1/2 -translate-x-1/2 -translate-y-1/2 w-8 h-8 rounded-full bg-white border-2 border-primary shadow-lg flex items-center justify-center pointer-events-none">
+            <svg
+              width="14"
+              height="14"
+              viewBox="0 0 14 14"
+              fill="none"
+              className="text-primary"
+            >
+              <path
+                d="M4 3L1 7L4 11"
+                stroke="currentColor"
+                strokeWidth="1.5"
+                strokeLinecap="round"
+                strokeLinejoin="round"
+              />
+              <path
+                d="M10 3L13 7L10 11"
+                stroke="currentColor"
+                strokeWidth="1.5"
+                strokeLinecap="round"
+                strokeLinejoin="round"
+              />
+            </svg>
+          </div>
+        </div>
+
+        {/* Labels */}
+        <div className="absolute top-2 left-2 px-2 py-0.5 rounded bg-black/50 text-white text-xs font-medium pointer-events-none">
+          Original
+        </div>
+        <div className="absolute top-2 right-2 px-2 py-0.5 rounded bg-black/50 text-white text-xs font-medium pointer-events-none">
+          Processed
+        </div>
+      </div>
+
+      {/* Size comparison badges */}
+      {beforeSize != null && afterSize != null && (
+        <div className="flex items-center gap-4 text-xs">
+          <span className="px-2 py-1 rounded bg-muted text-muted-foreground">
+            Original: {formatSize(beforeSize)}
+          </span>
+          <span className="px-2 py-1 rounded bg-primary/10 text-primary font-medium">
+            Processed: {formatSize(afterSize)}
+            {savingsPercent !== null && Number(savingsPercent) > 0 && (
+              <span className="ml-1">({savingsPercent}% smaller)</span>
+            )}
+            {savingsPercent !== null && Number(savingsPercent) < 0 && (
+              <span className="ml-1">
+                ({Math.abs(Number(savingsPercent))}% larger)
+              </span>
+            )}
+          </span>
+        </div>
+      )}
+    </div>
+  );
+}

apps/web/src/components/common/dropzone.tsx

@@ -1,14 +1,16 @@
 import { useCallback, useState, type DragEvent } from "react";
-import { Upload } from "lucide-react";
+import { Upload, FileImage } from "lucide-react";
 import { cn } from "@/lib/utils";
 
 interface DropzoneProps {
   onFiles?: (files: File[]) => void;
   accept?: string;
   multiple?: boolean;
+  /** Files that have already been dropped (for showing count & list). */
+  currentFiles?: File[];
 }
 
-export function Dropzone({ onFiles, accept, multiple = true }: DropzoneProps) {
+export function Dropzone({ onFiles, accept, multiple = true, currentFiles = [] }: DropzoneProps) {
   const [isDragging, setIsDragging] = useState(false);
 
   const handleDrag = useCallback((e: DragEvent) => {
@@ -41,6 +43,8 @@ export function Dropzone({ onFiles, accept, multiple = true }: DropzoneProps) {
     input.click();
   };
 
+  const hasMultipleFiles = currentFiles.length > 1;
+
   return (
     <div
       onDragEnter={handleDrag}
@@ -66,6 +70,27 @@ export function Dropzone({ onFiles, accept, multiple = true }: DropzoneProps) {
         <p className="text-sm text-muted-foreground">
           Drop files here or click the upload button
         </p>
+
+        {/* Show file count badge and list when multiple files are dropped */}
+        {hasMultipleFiles && (
+          <div className="flex flex-col items-center gap-2 mt-2">
+            <span className="inline-flex items-center gap-1.5 px-3 py-1 rounded-full bg-primary/10 text-primary text-xs font-medium">
+              <FileImage className="h-3.5 w-3.5" />
+              {currentFiles.length} files selected
+            </span>
+            <div className="max-h-32 overflow-y-auto w-full max-w-xs">
+              {currentFiles.map((f, i) => (
+                <div
+                  key={i}
+                  className="flex items-center justify-between text-xs text-muted-foreground px-2 py-0.5"
+                >
+                  <span className="truncate">{f.name}</span>
+                  <span className="shrink-0 ml-2">{(f.size / 1024).toFixed(0)} KB</span>
+                </div>
+              ))}
+            </div>
+          </div>
+        )}
       </div>
     </div>
   );

apps/web/src/pages/tool-page.tsx

@@ -3,6 +3,7 @@ import { useMemo, useCallback } from "react";
 import { TOOLS } from "@stirling-image/shared";
 import { AppLayout } from "@/components/layout/app-layout";
 import { Dropzone } from "@/components/common/dropzone";
+import { BeforeAfterSlider } from "@/components/common/before-after-slider";
 import { useFileStore } from "@/stores/file-store";
 import { ResizeSettings } from "@/components/tools/resize-settings";
 import { CropSettings } from "@/components/tools/crop-settings";
@@ -39,7 +40,7 @@ function ToolSettingsPanel({ toolId }: { toolId: string }) {
 export function ToolPage() {
   const { toolId } = useParams<{ toolId: string }>();
   const tool = useMemo(() => TOOLS.find((t) => t.id === toolId), [toolId]);
-  const { files, setFiles, reset } = useFileStore();
+  const { files, setFiles, reset, processedUrl, originalBlobUrl, originalSize, processedSize } = useFileStore();
 
   const handleFiles = useCallback(
     (newFiles: File[]) => {
@@ -128,11 +129,21 @@ export function ToolPage() {
 
         {/* Dropzone / Preview */}
         <div className="flex-1 flex items-center justify-center p-6">
-          <Dropzone
-            onFiles={handleFiles}
-            accept="image/*"
-            multiple={false}
-          />
+          {processedUrl && originalBlobUrl ? (
+            <BeforeAfterSlider
+              beforeSrc={originalBlobUrl}
+              afterSrc={processedUrl}
+              beforeSize={originalSize ?? undefined}
+              afterSize={processedSize ?? undefined}
+            />
+          ) : (
+            <Dropzone
+              onFiles={handleFiles}
+              accept="image/*"
+              multiple
+              currentFiles={files}
+            />
+          )}
         </div>
       </div>
     </AppLayout>

apps/web/src/stores/file-store.ts

@@ -4,6 +4,8 @@ interface FileState {
   files: File[];
   jobId: string | null;
   processedUrl: string | null;
+  /** Blob URL for the original image (for before/after comparison). */
+  originalBlobUrl: string | null;
   processing: boolean;
   error: string | null;
   originalSize: number | null;
@@ -11,35 +13,49 @@ interface FileState {
   setFiles: (files: File[]) => void;
   setJobId: (id: string) => void;
   setProcessedUrl: (url: string | null) => void;
+  setOriginalBlobUrl: (url: string | null) => void;
   setProcessing: (v: boolean) => void;
   setError: (e: string | null) => void;
   setSizes: (original: number, processed: number) => void;
   reset: () => void;
 }
 
-export const useFileStore = create<FileState>((set) => ({
+export const useFileStore = create<FileState>((set, get) => ({
   files: [],
   jobId: null,
   processedUrl: null,
+  originalBlobUrl: null,
   processing: false,
   error: null,
   originalSize: null,
   processedSize: null,
-  setFiles: (files) => set({ files, error: null }),
+  setFiles: (files) => {
+    // Revoke old blob URL if any
+    const old = get().originalBlobUrl;
+    if (old) URL.revokeObjectURL(old);
+    // Create a blob URL for the first file for before/after preview
+    const blobUrl = files.length > 0 ? URL.createObjectURL(files[0]) : null;
+    set({ files, error: null, originalBlobUrl: blobUrl });
+  },
   setJobId: (id) => set({ jobId: id }),
   setProcessedUrl: (url) => set({ processedUrl: url }),
+  setOriginalBlobUrl: (url) => set({ originalBlobUrl: url }),
   setProcessing: (v) => set({ processing: v }),
   setError: (e) => set({ error: e, processing: false }),
   setSizes: (original, processed) =>
     set({ originalSize: original, processedSize: processed }),
-  reset: () =>
+  reset: () => {
+    const old = get().originalBlobUrl;
+    if (old) URL.revokeObjectURL(old);
     set({
       files: [],
       jobId: null,
       processedUrl: null,
+      originalBlobUrl: null,
       processing: false,
       error: null,
       originalSize: null,
       processedSize: null,
-    }),
+    });
+  },
 }));