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/download.ts

@@ -0,0 +1,29 @@
+/** Trigger a client-side file download from a Blob. */
+export function downloadBlob(filename: string, blob: Blob): void {
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  a.remove();
+  // revoke on the next tick so the download has started
+  setTimeout(() => URL.revokeObjectURL(url), 1000);
+}
+
+export function downloadText(
+  filename: string,
+  text: string,
+  mime = 'text/plain;charset=utf-8',
+): void {
+  downloadBlob(filename, new Blob([text], { type: mime }));
+}
+
+export function downloadDataUrl(filename: string, dataUrl: string): void {
+  const a = document.createElement('a');
+  a.href = dataUrl;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  a.remove();
+}