Add Rails backend: accounts via magic link, server documents, sharing

The app becomes a Rails 8 application; the Vue frontend moves to
app/frontend and is served through vite_rails.

- Without an account documents stay in the browser (IndexedDB). Sign-in
  is by magic link or the 6-digit code from the same mail; afterwards
  local documents can be uploaded into the account.
- Server documents (/d/<id>) with versioned saves and a single-editor
  lock per browser tab; others see them read-only and follow by polling.
- Sharing via link (works without an account) and via e-mail invitation,
  roles viewer/editor; the owner manages links and people.
- Store gains a read-only mode that blocks all content changes.
- Solid Queue, Cache and Cable share the one PostgreSQL database.

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

package.json

@@ -5,14 +5,13 @@
   "type": "module",
   "description": "Browser-Tabellenkalkulation nach dem Vorbild von Apple Numbers – mit selbst geschriebener Formel-Engine.",
   "scripts": {
-    "dev": "vite --port 3000",
+    "dev": "bin/dev",
     "build": "vue-tsc --noEmit && vite build",
-    "preview": "vite preview",
     "test": "vitest run",
     "test:watch": "vitest",
+    "test:server": "bin/rails test",
     "lint": "eslint . --ext .ts,.vue",
-    "format": "prettier --write \"src/**/*.{ts,vue,css}\"",
-    "sync": "rsync -aPv dist/* wkh@rails.wicom.li:/home/www/numolus.wicom.li/htdocs/"
+    "format": "prettier --write \"app/frontend/**/*.{ts,vue,css}\""
   },
   "dependencies": {
     "chart.js": "^4.4.6",
@@ -30,7 +29,8 @@
     "jsdom": "^25.0.1",
     "prettier": "^3.3.3",
     "typescript": "~5.6.3",
-    "vite": "^5.4.11",
+    "vite": "^5.4.21",
+    "vite-plugin-ruby": "^5.2.4",
     "vitest": "^2.1.5",
     "vue-tsc": "^2.1.10"
   }