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/io/__tests__/library.spec.ts

@@ -0,0 +1,64 @@
+import { describe, expect, it } from 'vitest';
+import { IDBFactory } from 'fake-indexeddb';
+import { DocumentLibrary, newDocId, type DocMeta } from '../library';
+
+const meta = (id: string, title: string, updatedAt: number): DocMeta => ({
+  id,
+  title,
+  createdAt: 1,
+  updatedAt,
+});
+
+/** a fresh, empty in-memory IndexedDB per test */
+const openLib = () => DocumentLibrary.open(new IDBFactory());
+
+describe('DocumentLibrary', () => {
+  it('stores meta and content separately and reads them back', async () => {
+    const lib = await openLib();
+    await lib.put(meta('a', 'Budget', 10), '{"x":1}');
+    expect(await lib.getMeta('a')).toEqual(meta('a', 'Budget', 10));
+    expect(await lib.getContent('a')).toBe('{"x":1}');
+    expect(await lib.getContent('missing')).toBeUndefined();
+  });
+
+  it('lists documents most recently changed first', async () => {
+    const lib = await openLib();
+    await lib.put(meta('old', 'Alt', 10), '{}');
+    await lib.put(meta('new', 'Neu', 30), '{}');
+    await lib.put(meta('mid', 'Mitte', 20), '{}');
+    expect((await lib.list()).map((d) => d.id)).toEqual(['new', 'mid', 'old']);
+  });
+
+  it('patchMeta renames without touching content; unknown ids are ignored', async () => {
+    const lib = await openLib();
+    await lib.put(meta('a', 'Alt', 10), '{"keep":true}');
+    await lib.patchMeta('a', { title: 'Neu' });
+    await lib.patchMeta('nope', { title: 'X' });
+    expect((await lib.getMeta('a'))?.title).toBe('Neu');
+    expect(await lib.getContent('a')).toBe('{"keep":true}');
+    expect(await lib.list()).toHaveLength(1);
+  });
+
+  it('remove deletes meta and content', async () => {
+    const lib = await openLib();
+    await lib.put(meta('a', 'A', 10), '{}');
+    await lib.remove('a');
+    expect(await lib.getMeta('a')).toBeUndefined();
+    expect(await lib.getContent('a')).toBeUndefined();
+    expect(await lib.list()).toEqual([]);
+  });
+
+  it('keeps data across re-opening the same database', async () => {
+    const factory = new IDBFactory();
+    await (await DocumentLibrary.open(factory)).put(meta('a', 'A', 10), '{"v":2}');
+    expect(await (await DocumentLibrary.open(factory)).getContent('a')).toBe('{"v":2}');
+  });
+});
+
+describe('newDocId', () => {
+  it('produces distinct v4 UUIDs', () => {
+    const a = newDocId();
+    expect(a).toMatch(/^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/);
+    expect(newDocId()).not.toBe(a);
+  });
+});