feat: implement reference image comparison tools and upload functionality

IMPLEMENTATION_PLAN.md

@@ -290,6 +290,70 @@ src/
 - Excel export (multi-sheet styled workbook)
 - Deadline tracking (approaching/overdue detection + notifications)
 
+### Phase 3b: Reference Image Comparison (Markup Review)
+- **Reference image upload** — attach reference/approved images to any revision round. Stored via the existing `attachments` JSON field on `Revision`, with file upload to local storage (Phase 1) or S3/Cloud (production). Supports PNG, JPG, WebP up to 10MB.
+- **Current render upload** — upload the current WIP render alongside the reference for direct comparison. Each revision round can have a "reference" image and a "current" image.
+- **A-B comparison slider** — draggable vertical divider overlaying reference (left) and current (right) images. User drags to reveal/hide each side. Built as a `<ImageCompareSlider>` component using pointer events + CSS clip-path. No external dependency needed.
+- **Toggle mode** — click/tap to flip between reference and current image (full-frame swap with crossfade transition). Keyboard shortcut: `Space` to toggle. Useful on smaller screens where the slider is impractical.
+- **Side-by-side mode** — display both images at equal size in a horizontal split. Synced zoom/pan so both images move together. Default on wide screens.
+- **Zoom & pan** — scroll-to-zoom with pinch support. Pan by click-drag when zoomed. Zoom level synced across both images in all comparison modes.
+- **Image gallery per revision** — revision rounds accumulate images over time. Gallery shows all uploaded images for the stage across rounds, with round number labels. Quick-select any past round's image as the reference for comparison.
+- **Context in StageDetailSheet** — new "Compare" tab in the stage detail sheet (alongside existing Revisions and Comments tabs). Comparison tools live here. Selecting a revision round auto-loads its images.
+- **Lightweight markup overlay** (optional enhancement) — basic annotation tools (arrow, rectangle, freehand, text pin) drawn on top of the current image. Annotations saved as JSON on the revision. Not a full drawing app — just enough to point at problems.
+
+#### Data Model Changes
+```
+// Extend Revision.attachments JSON schema:
+{
+  referenceImage?: { url: string, filename: string, size: number, uploadedAt: string },
+  currentImage?: { url: string, filename: string, size: number, uploadedAt: string },
+  annotations?: Array<{ type: "arrow"|"rect"|"freehand"|"pin", points: number[], text?: string, color: string }>
+}
+```
+
+#### New API Endpoints
+```
+POST /api/stages/:stageId/revisions/:revisionId/upload   — multipart form upload (reference or current image)
+DELETE /api/stages/:stageId/revisions/:revisionId/upload  — remove an uploaded image
+```
+
+#### New Components
+```
+src/components/revisions/
+├── image-compare-slider.tsx    — A-B slider with drag handle
+├── image-toggle.tsx            — Toggle/crossfade between two images
+├── image-side-by-side.tsx      — Synced side-by-side with zoom/pan
+├── image-upload-zone.tsx       — Drag-and-drop upload for reference/current
+├── revision-image-gallery.tsx  — Browse images across revision rounds
+├── compare-tab.tsx             — Orchestrator: mode selector + comparison view
+└── markup-overlay.tsx          — Lightweight annotation layer (optional)
+```
+
+#### PNG Transparency Handling
+CG renders are typically PNGs with transparent backgrounds and semi-transparent drop shadows. When two transparent PNGs are layered in comparison modes (slider, toggle, overlay), the shadows multiply and produce incorrect visual results — making review unreliable.
+
+**Solution: server-side alpha compositing on upload.**
+- On the upload endpoint, detect if the image is a PNG with an alpha channel (check IHDR color type via `sharp` or raw buffer inspection).
+- If alpha is present, composite the image onto a solid **white (#FFFFFF) background** using `sharp.flatten({ background: '#FFFFFF' })` before storing.
+- Store the flattened version as the comparison-ready image. Optionally keep the original transparent PNG as a separate `originalUrl` for download purposes.
+- JPG and WebP uploads (no alpha) skip this step — no processing needed.
+- This happens once at upload time, not at render time — avoids per-view processing cost and ensures all comparison modes work correctly without CSS hacks.
+
+```
+// Upload processing pipeline (pseudo):
+const img = sharp(buffer);
+const metadata = await img.metadata();
+if (metadata.format === 'png' && metadata.hasAlpha) {
+  buffer = await img.flatten({ background: '#FFFFFF' }).png().toBuffer();
+}
+```
+
+#### UX Notes
+- Images are almost never reviewed in isolation — the reference comparison is the primary review workflow. The compare tab should be the default/first tab when images exist.
+- Default to slider mode on desktop, toggle mode on mobile.
+- Reference images persist across rounds (the approved reference doesn't change until a new one is set). Current images update each round.
+- Empty state: prompt to upload a reference image with a clear CTA. Show how comparison works with a placeholder illustration.
+
 ### Phase 4: Polish
 - Command palette (Cmd+K search)
 - Bulk operations (multi-select in table → bulk status/assignment/priority)