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/engine/__tests__/functions.spec.ts

@@ -0,0 +1,18 @@
+import { describe, it, expect } from 'vitest';
+import { FUNCTIONS, BUILTIN_SIGNATURES } from '../functions';
+
+describe('BUILTIN_SIGNATURES', () => {
+  it('has exactly one entry per registered built-in function, names matching', () => {
+    const registered = Object.keys(FUNCTIONS).sort();
+    const signatures = BUILTIN_SIGNATURES.map((s) => s.name).sort();
+    // A future new built-in without a matching hint entry fails loudly here
+    // instead of just silently being missing from the autocomplete dropdown.
+    expect(signatures).toEqual(registered);
+  });
+
+  it('every hint actually starts with its own name', () => {
+    for (const s of BUILTIN_SIGNATURES) {
+      expect(s.hint.startsWith(s.name)).toBe(true);
+    }
+  });
+});