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/frontend/entrypoints/application.ts

@@ -0,0 +1,10 @@
+import { createApp } from 'vue';
+import App from '../App.vue';
+import { useLibrary } from '../store/library';
+import '../styles/main.css';
+
+// open the document (from IndexedDB) before the first render, so the app
+// never flashes an empty document first
+void useLibrary()
+  .init()
+  .finally(() => createApp(App).mount('#app'));