feat: add adjustments, filters, levels, curves, histogram, and effects panels for image editor

Implements Features 17, 18, 35, 36, 41, 42, 43 for the editor:
- SliderRow reusable component (label + range + numeric input)
- AdjustmentsPanel with 8 adjustment sliders (brightness, contrast,
  hue, saturation, luminance, exposure, vibrance, warmth)
- Auto adjustments: Auto Tone, Auto Contrast, Auto Color, Auto Enhance
- Levels section with per-channel control, histogram display, black/white
  point, gamma, and output range sliders
- Curves section with 200x200 interactive graph, cubic spline
  interpolation, per-channel support, and 9 presets
- 12 filter controls: toggle (grayscale, sepia, invert, solarize) and
  slider (blur, sharpen, noise, pixelate, emboss, posterize, threshold,
  kaleidoscope)
- Additional blur types: motion blur, radial blur, surface blur
- Vignette with amount/midpoint/roundness/feather sliders
- Grain with amount/size/roughness sliders
- HistogramPanel with RGB channel overlays and stats (mean, stddev, median)
- Reset All and Apply action buttons
- 150ms debounce on adjustment sliders, 300ms on histogram updates

apps/web/src/components/editor/common/slider-row.tsx

@@ -0,0 +1,52 @@
+// apps/web/src/components/editor/common/slider-row.tsx
+
+import { cn } from "@/lib/utils";
+
+interface SliderRowProps {
+  label: string;
+  value: number;
+  min: number;
+  max: number;
+  step?: number;
+  onChange: (value: number) => void;
+  className?: string;
+}
+
+export function SliderRow({
+  label,
+  value,
+  min,
+  max,
+  step = 1,
+  onChange,
+  className,
+}: SliderRowProps) {
+  return (
+    <div className={cn("flex items-center gap-2", className)}>
+      <span className="text-xs text-muted-foreground w-20 shrink-0 truncate">{label}</span>
+      <input
+        type="range"
+        min={min}
+        max={max}
+        step={step}
+        value={value}
+        onChange={(e) => onChange(Number(e.target.value))}
+        className="flex-1 h-1 accent-primary cursor-pointer"
+      />
+      <input
+        type="number"
+        min={min}
+        max={max}
+        step={step}
+        value={value}
+        onChange={(e) => {
+          const v = Number(e.target.value);
+          if (!Number.isNaN(v)) {
+            onChange(Math.max(min, Math.min(max, v)));
+          }
+        }}
+        className="w-14 px-1 py-0.5 text-xs text-right bg-muted border border-border rounded text-foreground"
+      />
+    </div>
+  );
+}

apps/web/src/components/editor/panels/histogram-panel.tsx

@@ -0,0 +1,206 @@
+// apps/web/src/components/editor/panels/histogram-panel.tsx
+
+import { useCallback, useEffect, useRef, useState } from "react";
+
+const HIST_WIDTH = 256;
+const HIST_HEIGHT = 80;
+const DEBOUNCE_MS = 300;
+
+interface HistogramStats {
+  mean: [number, number, number];
+  stdDev: [number, number, number];
+  median: [number, number, number];
+}
+
+function computeHistogram(imageData: ImageData) {
+  const rBins = new Uint32Array(256);
+  const gBins = new Uint32Array(256);
+  const bBins = new Uint32Array(256);
+  const { data } = imageData;
+
+  for (let i = 0; i < data.length; i += 4) {
+    rBins[data[i]]++;
+    gBins[data[i + 1]]++;
+    bBins[data[i + 2]]++;
+  }
+
+  return { rBins, gBins, bBins };
+}
+
+function computeStats(rBins: Uint32Array, gBins: Uint32Array, bBins: Uint32Array): HistogramStats {
+  const channelStats = (bins: Uint32Array): [number, number, number] => {
+    let total = 0;
+    let sum = 0;
+    let sumSq = 0;
+
+    for (let i = 0; i < 256; i++) {
+      total += bins[i];
+      sum += i * bins[i];
+      sumSq += i * i * bins[i];
+    }
+
+    if (total === 0) return [0, 0, 0];
+
+    const mean = sum / total;
+    const variance = sumSq / total - mean * mean;
+    const stdDev = Math.sqrt(Math.max(0, variance));
+
+    // Median
+    let cumulative = 0;
+    const half = total / 2;
+    let median = 0;
+    for (let i = 0; i < 256; i++) {
+      cumulative += bins[i];
+      if (cumulative >= half) {
+        median = i;
+        break;
+      }
+    }
+
+    return [Math.round(mean * 10) / 10, Math.round(stdDev * 10) / 10, median];
+  };
+
+  const [rMean, rStdDev, rMedian] = channelStats(rBins);
+  const [gMean, gStdDev, gMedian] = channelStats(gBins);
+  const [bMean, bStdDev, bMedian] = channelStats(bBins);
+
+  return {
+    mean: [rMean, gMean, bMean],
+    stdDev: [rStdDev, gStdDev, bStdDev],
+    median: [rMedian, gMedian, bMedian],
+  };
+}
+
+function drawChannel(
+  ctx: CanvasRenderingContext2D,
+  bins: Uint32Array,
+  maxVal: number,
+  color: string,
+) {
+  if (maxVal === 0) return;
+
+  ctx.fillStyle = color;
+  ctx.globalAlpha = 0.4;
+  ctx.beginPath();
+  ctx.moveTo(0, HIST_HEIGHT);
+
+  for (let i = 0; i < 256; i++) {
+    const h = (bins[i] / maxVal) * HIST_HEIGHT;
+    ctx.lineTo(i, HIST_HEIGHT - h);
+  }
+
+  ctx.lineTo(255, HIST_HEIGHT);
+  ctx.closePath();
+  ctx.fill();
+
+  ctx.strokeStyle = color;
+  ctx.globalAlpha = 0.7;
+  ctx.lineWidth = 1;
+  ctx.beginPath();
+
+  for (let i = 0; i < 256; i++) {
+    const h = (bins[i] / maxVal) * HIST_HEIGHT;
+    if (i === 0) {
+      ctx.moveTo(i, HIST_HEIGHT - h);
+    } else {
+      ctx.lineTo(i, HIST_HEIGHT - h);
+    }
+  }
+
+  ctx.stroke();
+  ctx.globalAlpha = 1;
+}
+
+interface HistogramPanelProps {
+  stageRef?: React.RefObject<HTMLCanvasElement | null>;
+  imageData?: ImageData | null;
+}
+
+export function HistogramPanel({ imageData }: HistogramPanelProps) {
+  const canvasRef = useRef<HTMLCanvasElement>(null);
+  const [stats, setStats] = useState<HistogramStats | null>(null);
+  const debounceRef = useRef<ReturnType<typeof setTimeout>>(null);
+
+  const renderHistogram = useCallback((data: ImageData) => {
+    const canvas = canvasRef.current;
+    if (!canvas) return;
+
+    const ctx = canvas.getContext("2d");
+    if (!ctx) return;
+
+    const { rBins, gBins, bBins } = computeHistogram(data);
+    const newStats = computeStats(rBins, gBins, bBins);
+    setStats(newStats);
+
+    // Find max for normalization
+    let maxVal = 0;
+    for (let i = 0; i < 256; i++) {
+      maxVal = Math.max(maxVal, rBins[i], gBins[i], bBins[i]);
+    }
+
+    ctx.clearRect(0, 0, HIST_WIDTH, HIST_HEIGHT);
+
+    // Background
+    ctx.fillStyle = "rgba(0, 0, 0, 0.15)";
+    ctx.fillRect(0, 0, HIST_WIDTH, HIST_HEIGHT);
+
+    drawChannel(ctx, rBins, maxVal, "rgba(239, 68, 68, 1)");
+    drawChannel(ctx, gBins, maxVal, "rgba(34, 197, 94, 1)");
+    drawChannel(ctx, bBins, maxVal, "rgba(59, 130, 246, 1)");
+  }, []);
+
+  useEffect(() => {
+    if (!imageData) return;
+
+    if (debounceRef.current) {
+      clearTimeout(debounceRef.current);
+    }
+
+    debounceRef.current = setTimeout(() => {
+      renderHistogram(imageData);
+    }, DEBOUNCE_MS);
+
+    return () => {
+      if (debounceRef.current) {
+        clearTimeout(debounceRef.current);
+      }
+    };
+  }, [imageData, renderHistogram]);
+
+  return (
+    <div className="flex flex-col gap-1.5">
+      <canvas
+        ref={canvasRef}
+        width={HIST_WIDTH}
+        height={HIST_HEIGHT}
+        className="w-full rounded border border-border"
+        style={{ imageRendering: "pixelated" }}
+      />
+      {stats && (
+        <div className="grid grid-cols-3 gap-1 text-[10px] text-muted-foreground">
+          <div>
+            <span className="font-medium">Mean</span>
+            <br />
+            <span className="text-red-400">{stats.mean[0]}</span>{" "}
+            <span className="text-green-400">{stats.mean[1]}</span>{" "}
+            <span className="text-blue-400">{stats.mean[2]}</span>
+          </div>
+          <div>
+            <span className="font-medium">StdDev</span>
+            <br />
+            <span className="text-red-400">{stats.stdDev[0]}</span>{" "}
+            <span className="text-green-400">{stats.stdDev[1]}</span>{" "}
+            <span className="text-blue-400">{stats.stdDev[2]}</span>
+          </div>
+          <div>
+            <span className="font-medium">Median</span>
+            <br />
+            <span className="text-red-400">{stats.median[0]}</span>{" "}
+            <span className="text-green-400">{stats.median[1]}</span>{" "}
+            <span className="text-blue-400">{stats.median[2]}</span>
+          </div>
+        </div>
+      )}
+    </div>
+  );
+}