Live collaboration with Yjs over ActionCable

All editors now work on a server document at the same time instead of
taking turns behind an edit lock.

- The document's content is a sequence of Yjs updates (document_updates);
  DocumentChannel stores and relays them immediately, checks access on
  subscribe and again whenever links or memberships change.
- app/frontend/collab/: the store stays position-based; a binding maps it
  to a Yjs form with stable row/column ids, so a row inserted by one
  person doesn't misplace what another is typing. Remote changes are
  applied only after pending local ones went into Yjs.
- Per-person undo (Y.UndoManager), presence via awareness: the others'
  selections as coloured boxes with names, avatars in the toolbar.
- Existing JSON documents convert on first open, deterministically, so
  two people opening one at once end up with the same Yjs state.
- Editors compact the stored updates after 200 and keep a JSON snapshot
  current for export and duplicating. Edit lock and polling are gone.

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

package.json

@@ -14,12 +14,17 @@
     "format": "prettier --write \"app/frontend/**/*.{ts,vue,css}\""
   },
   "dependencies": {
+    "@rails/actioncable": "^8.1.400",
     "chart.js": "^4.4.6",
     "fflate": "^0.8.3",
+    "lib0": "^0.2.119",
     "vue": "^3.5.13",
-    "xlsx": "^0.18.5"
+    "xlsx": "^0.18.5",
+    "y-protocols": "^1.0.7",
+    "yjs": "^13.6.33"
   },
   "devDependencies": {
+    "@types/rails__actioncable": "^8.0.3",
     "@vitejs/plugin-vue": "^5.2.0",
     "@vue/eslint-config-prettier": "^10.1.0",
     "@vue/eslint-config-typescript": "^14.1.3",