feat: overhaul remove-background with effects pipeline, consolidate color tools

Remove Background:
- Two-phase flow: AI removes bg once, then effects adjust instantly
- Blur background effect with real-time CSS preview (portrait mode)
- Drop shadow effect with opacity control
- Gradient backgrounds with presets, custom colors, and angle
- Custom background image upload (including HEIC/HEIF)
- Solid color backgrounds moved from Python to Node.js/Sharp
- Effects-only API endpoint for instant re-renders without AI re-run
- HEIC/HEIF input support (decoded before passing to Python/rembg)
- Passport/ID photo checkbox defaults ON for People subject
- Before/after slider preserved when no effects active
- 15 comprehensive Playwright e2e tests

Color Tools:
- Consolidated 4 tools (brightness-contrast, saturation, color-channels,
  color-effects) into single "Adjust Colors" tool
- Added exposure, temperature, tint, hue, sharpness controls
- SVG filter-based live preview for all adjustments
- Backward-compatible URL redirects from old tool paths

Other fixes:
- Favicon tool: download button instead of auto-download
- Batch processing: HEIC filename extension fix
- File store: processedFilename field for proper batch downloads

apps/web/src/hooks/use-tool-processor.ts

@@ -124,10 +124,17 @@ export function useToolProcessor(toolId: string) {
         }
       }
 
-      // Build form data
+      // Build form data - extract any File objects from settings before JSON serialization
+      const cleanSettings = { ...settings };
+      const bgImageFile = cleanSettings._bgImageFile as File | undefined;
+      delete cleanSettings._bgImageFile;
+
       const formData = new FormData();
       formData.append("file", files[0]);
-      formData.append("settings", JSON.stringify(settings));
+      formData.append("settings", JSON.stringify(cleanSettings));
+      if (bgImageFile) {
+        formData.append("backgroundImage", bgImageFile);
+      }
       if (isAiTool) {
         formData.append("clientJobId", clientJobId);
       }
@@ -368,6 +375,7 @@ export function useToolProcessor(toolId: string) {
             const blob = new Blob([extracted[processedName] as BlobPart]);
             updateEntry(i, {
               processedUrl: URL.createObjectURL(blob),
+              processedFilename: processedName,
               processedSize: blob.size,
               status: "completed",
               error: null,