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>

app/views/pwa/manifest.json.erb

@@ -0,0 +1,22 @@
+{
+  "name": "Numolus",
+  "icons": [
+    {
+      "src": "/icon.png",
+      "type": "image/png",
+      "sizes": "512x512"
+    },
+    {
+      "src": "/icon.png",
+      "type": "image/png",
+      "sizes": "512x512",
+      "purpose": "maskable"
+    }
+  ],
+  "start_url": "/",
+  "display": "standalone",
+  "scope": "/",
+  "description": "Numolus.",
+  "theme_color": "red",
+  "background_color": "red"
+}