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/api/src/routes/batch.ts

@@ -146,15 +146,19 @@ export async function registerBatchRoutes(app: FastifyInstance): Promise<void> {
 
             try {
               let processBuffer = file.buffer;
+              let processFilename = file.filename;
               // Skip HEIC decode and auto-orient for edit-metadata (ExifTool handles all formats natively)
               const skipPreprocess = toolId === "edit-metadata" || toolId === "strip-metadata";
               if (!skipPreprocess && validation.format === "heif") {
                 processBuffer = await decodeHeic(processBuffer);
+                // Update extension to match decoded format (HEIC/HEIF → PNG)
+                const ext = processFilename.match(/\.[^.]+$/)?.[0];
+                if (ext) processFilename = processFilename.slice(0, -ext.length) + ".png";
               }
               if (!skipPreprocess) {
                 processBuffer = await autoOrient(processBuffer);
               }
-              const result = await toolConfig.process(processBuffer, settings, file.filename);
+              const result = await toolConfig.process(processBuffer, settings, processFilename);
 
               results[index] = { buffer: result.buffer, filename: result.filename };