feat(notes): browser sync engine with offline queue; editor saves through the operation log

Phase 2 of the collaboration plan:
- frontend/src/lib/docSync: TypeScript port of the block tree, operations,
  effects and order keys (kept identical to Go via shared test vectors),
  diff from the editor's flat list to operations, IndexedDB persistence,
  HTTP transport and DocSyncEngine (one request in flight, idempotent
  resends, backoff, 3 s/30 s polling, keepalive flush and leave on exit,
  block leases for focus, 20 s grace and new blocks).
- Text changes carry the version the user saw when typing (first change
  wins when coalescing; a stale focused field keeps its old base), so a
  concurrent edit becomes a conflict copy instead of a silent overwrite.
- DocumentEditor uses the engine instead of the 800 ms whole-document PUT;
  editors (not only owners) can edit, remote changes appear live.
- Server: task IDs of linked blocks are delivered again, linked tasks follow
  ticked/deleted blocks after sync, note tags are derived, title changes
  notify other users; block IDs already used in another note are renamed
  (starter content with fixed IDs was empty for every further user).
- Fix: unlinking tasks from blocks wrote to non-existent columns and never
  worked (error was swallowed).
- Tests: Go/TS order key vectors, diff round trips, engine races (lost
  responses, offline + reload, edits arriving between typing and sending,
  stale focused field, duplicate first block), plus a Playwright test with
  two browsers against the real image (e2e/run.sh).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/package-lock.json

@@ -22,6 +22,7 @@
         "@types/react-dom": "^18.3.1",
         "@vitejs/plugin-react": "^4.3.4",
         "autoprefixer": "^10.4.20",
+        "fake-indexeddb": "^6.2.6",
         "jsdom": "^25.0.1",
         "postcss": "^8.4.49",
         "tailwindcss": "^3.4.17",
@@ -2238,6 +2239,16 @@
         "node": ">=12.0.0"
       }
     },
+    "node_modules/fake-indexeddb": {
+      "version": "6.2.6",
+      "resolved": "https://registry.npmjs.org/fake-indexeddb/-/fake-indexeddb-6.2.6.tgz",
+      "integrity": "sha512-ihhmr0KoET51jT8R4YhbUYuLLvr1qqO6INISygtN16i8loR8kAxtAUr4+EKHTSMjL+lggaAn4OC7DRqzrNgnuQ==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/fast-glob": {
       "version": "3.3.3",
       "resolved": "https://registry.npmjs.org/fast-glob/-/fast-glob-3.3.3.tgz",