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

packages/shared/src/constants.ts

@@ -97,36 +97,12 @@ export const TOOLS: Tool[] = [
   },
   // Adjustments
   {
-    id: "brightness-contrast",
-    name: "Brightness & Contrast",
-    description: "Adjust brightness and contrast levels",
+    id: "adjust-colors",
+    name: "Adjust Colors",
+    description: "Brightness, contrast, exposure, saturation, temperature, sharpness, and effects",
     category: "adjustments",
-    icon: "Sun",
-    route: "/brightness-contrast",
-  },
-  {
-    id: "saturation",
-    name: "Saturation & Exposure",
-    description: "Adjust color saturation and exposure",
-    category: "adjustments",
-    icon: "Palette",
-    route: "/saturation",
-  },
-  {
-    id: "color-channels",
-    name: "Color Channels",
-    description: "Adjust individual R, G, B channels",
-    category: "adjustments",
-    icon: "CircleDot",
-    route: "/color-channels",
-  },
-  {
-    id: "color-effects",
-    name: "Color Effects",
-    description: "Grayscale, Sepia, Invert, Tint",
-    category: "adjustments",
-    icon: "Paintbrush",
-    route: "/color-effects",
+    icon: "SlidersHorizontal",
+    route: "/adjust-colors",
   },
   {
     id: "replace-color",