fix(markdown): polish editor UX — side panel, overlay sync, paste segregation

- Move markdown editor from canvas overlay to side panel (70% width)
  for better performance and editing experience
- Add @blocknote/mantine for full BlockNoteView with default UI components
- Fix socket reconnect loop caused by unstable pasteOpts object reference
  (memoize with useMemo)
- Add ResizeObserver to markdown overlay cards for automatic height sync
- Call mdOverlay.refreshAll() on every canvas change so overlays track
  pack/grid/arrange/save operations
- Paste goes to BlockNote editor when contentEditable is focused
- Simplify toolbar: single color picker (accent + auto-derived bg),
  title/name field, width presets S/M/L
- Guard normalize/flip operations to skip markdown and sticky items
- Fix title not updating on card preview (pass name prop, bump revision)
- Fix preview not refreshing after editor save (revision counter + overlay refresh)

frontend/package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@blocknote/core": "^0.47.1",
+    "@blocknote/mantine": "^0.47.1",
     "@blocknote/react": "^0.47.1",
     "axios": "^1.7.9",
     "pixi-filters": "^6.1.5",