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>

.gitignore

@@ -5,3 +5,25 @@ coverage
 .DS_Store
 .vscode/*
 !.vscode/extensions.json
+
+# Vite Ruby
+/public/vite*
+node_modules
+# Vite uses dotenv and suggests to ignore local-only env files. See
+# https://vite.dev/guide/env-and-mode.html#env-files
+*.local
+
+# Rails
+/.bundle
+/log/*
+/tmp/*
+!/log/.keep
+!/tmp/.keep
+!/tmp/pids/
+/tmp/pids/*
+!/tmp/pids/.keep
+/storage/*
+!/storage/.keep
+/config/master.key
+/.env*
+/vendor/gems