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-lock.json

@@ -8,12 +8,17 @@
       "name": "numolus",
       "version": "1.0.0",
       "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",
@@ -934,6 +939,12 @@
         "url": "https://opencollective.com/pkgr"
       }
     },
+    "node_modules/@rails/actioncable": {
+      "version": "8.1.400",
+      "resolved": "https://registry.npmjs.org/@rails/actioncable/-/actioncable-8.1.400.tgz",
+      "integrity": "sha512-Qffc3/I9xcpeaal12hmi9EPRV3hz9FvakUHBR7kL0jpZviWwO9dyaG8F1ltcjkHIdFxQ5QqzRyequAWHMq17xQ==",
+      "license": "MIT"
+    },
     "node_modules/@rollup/rollup-android-arm-eabi": {
       "version": "4.63.5",
       "resolved": "https://registry.npmjs.org/@rollup/rollup-android-arm-eabi/-/rollup-android-arm-eabi-4.63.5.tgz",
@@ -1298,6 +1309,13 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/rails__actioncable": {
+      "version": "8.0.3",
+      "resolved": "https://registry.npmjs.org/@types/rails__actioncable/-/rails__actioncable-8.0.3.tgz",
+      "integrity": "sha512-y46MOTYorVQVwlHUyaZYbrh3nIkXsRYNuPna32lb3RngLVBlndNbIPvAUywFfhivftNhYg+vW5sZKWYCVIX2lA==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@typescript-eslint/eslint-plugin": {
       "version": "8.68.0",
       "resolved": "https://registry.npmjs.org/@typescript-eslint/eslint-plugin/-/eslint-plugin-8.68.0.tgz",
@@ -3349,6 +3367,16 @@
       "dev": true,
       "license": "ISC"
     },
+    "node_modules/isomorphic.js": {
+      "version": "0.2.5",
+      "resolved": "https://registry.npmjs.org/isomorphic.js/-/isomorphic.js-0.2.5.tgz",
+      "integrity": "sha512-PIeMbHqMt4DnUP3MA/Flc0HElYjMXArsw1qwJZcm9sqR8mq3l8NYizFMty0pWwE/tzIGH3EKK5+jes5mAr85yw==",
+      "license": "MIT",
+      "funding": {
+        "type": "GitHub Sponsors ❤",
+        "url": "https://github.com/sponsors/dmonad"
+      }
+    },
     "node_modules/js-yaml": {
       "version": "4.3.2",
       "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-4.3.2.tgz",
@@ -3468,6 +3496,27 @@
         "node": ">= 0.8.0"
       }
     },
+    "node_modules/lib0": {
+      "version": "0.2.119",
+      "resolved": "https://registry.npmjs.org/lib0/-/lib0-0.2.119.tgz",
+      "integrity": "sha512-vh+TiejVy8Xm+hCdnRfn9BthGNvNUZdEXltNjVZL3TzRIKP3s1lnQh8pWd/Q0wCgY0FPAGVVjFF4ZiHUV3LFgA==",
+      "license": "MIT",
+      "dependencies": {
+        "isomorphic.js": "^0.2.4"
+      },
+      "bin": {
+        "0ecdsa-generate-keypair": "bin/0ecdsa-generate-keypair.js",
+        "0gentesthtml": "bin/gentesthtml.js",
+        "0serve": "bin/0serve.js"
+      },
+      "engines": {
+        "node": ">=16"
+      },
+      "funding": {
+        "type": "GitHub Sponsors ❤",
+        "url": "https://github.com/sponsors/dmonad"
+      }
+    },
     "node_modules/lightningcss": {
       "version": "1.33.0",
       "resolved": "https://registry.npmjs.org/lightningcss/-/lightningcss-1.33.0.tgz",
@@ -5146,6 +5195,43 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/y-protocols": {
+      "version": "1.0.7",
+      "resolved": "https://registry.npmjs.org/y-protocols/-/y-protocols-1.0.7.tgz",
+      "integrity": "sha512-YSVsLoXxO67J6eE/nV4AtFtT3QEotZf5sK5BHxFBXso7VDUT3Tx07IfA6hsu5Q5OmBdMkQVmFZ9QOA7fikWvnw==",
+      "license": "MIT",
+      "dependencies": {
+        "lib0": "^0.2.85"
+      },
+      "engines": {
+        "node": ">=16.0.0",
+        "npm": ">=8.0.0"
+      },
+      "funding": {
+        "type": "GitHub Sponsors ❤",
+        "url": "https://github.com/sponsors/dmonad"
+      },
+      "peerDependencies": {
+        "yjs": "^13.0.0"
+      }
+    },
+    "node_modules/yjs": {
+      "version": "13.6.33",
+      "resolved": "https://registry.npmjs.org/yjs/-/yjs-13.6.33.tgz",
+      "integrity": "sha512-q/UYvr1gOk4XII+Cu1CW2dINjYqJJFifTzogEaIARznrvX7uOlHsdTEXcnOBMuIsrCGEEKdNxi74kyMLZlKVfw==",
+      "license": "MIT",
+      "dependencies": {
+        "lib0": "^0.2.99"
+      },
+      "engines": {
+        "node": ">=16.0.0",
+        "npm": ">=8.0.0"
+      },
+      "funding": {
+        "type": "GitHub Sponsors ❤",
+        "url": "https://github.com/sponsors/dmonad"
+      }
+    },
     "node_modules/yocto-queue": {
       "version": "0.1.0",
       "resolved": "https://registry.npmjs.org/yocto-queue/-/yocto-queue-0.1.0.tgz",