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/image-engine/src/operations/sharpen.ts

@@ -0,0 +1,15 @@
+import type { Sharp, SharpenOptions } from "../types.js";
+
+export async function sharpen(image: Sharp, options: SharpenOptions): Promise<Sharp> {
+  const { value } = options;
+
+  if (value <= 0) return image;
+  if (value > 100) {
+    throw new Error("Sharpness value must be between 0 and 100");
+  }
+
+  // Map 0-100 to sigma 0.5-10
+  const sigma = 0.5 + (value / 100) * 9.5;
+
+  return image.sharpen({ sigma });
+}