feat: reusable visual PDF editor canvas (#6)

[nonamexishere]

Summary

Implements #6: reusable visual PDF editor canvas for later offline edit tools.

• Typed edit model + viewport ↔ PDF transforms (CropBox, page rotation)
• Undo/redo with gesture coalescing for move/resize
• Page surface (pdf.js) + SVG overlay: select / move / resize / delete draft rectangles
• Zoom uses stable stage fit-width (no blank page on zoom)
• Resize handles use PDF y-up anchors (correct opposite-corner behavior)
• Dev sandbox: sidebar “Editor canvas” (crop icon) → #/dev/editor-canvas
• Clear messaging when opened in browser-only Vite (needs npm run tauri:dev)
• Unit tests for coords, reducer, and resize

Out of scope (by design): writing objects into a PDF → issue #7.

How to test

1. npm install && npm run tauri:dev (desktop app — not browser-only)
2. Sidebar → Editor canvas (crop icon near Settings)
3. Add a PDF → draw/add rectangles → move, resize all corners, zoom, undo/redo
4. npm test && npm run build

Checklist

• Offline-only; no new network/telemetry dependencies
• Source PDF bytes not stored in the edit model
• npm test / npm run build pass
• Original files never written by this change (no PDF export yet)