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>

vite.config.ts

@@ -1,17 +1,19 @@
 /// <reference types="vitest/config" />
 import { defineConfig } from 'vite';
+import RubyPlugin from 'vite-plugin-ruby';
 import vue from '@vitejs/plugin-vue';
 
 export default defineConfig({
-  plugins: [vue()],
+  plugins: [RubyPlugin(), vue()],
   build: {
     target: 'es2020',
   },
+  // vite-plugin-ruby makes app/frontend the root, so paths are relative to it
   test: {
     environment: 'node',
-    include: ['src/**/*.spec.ts'],
+    include: ['**/*.spec.ts'],
     coverage: {
-      include: ['src/engine/**', 'src/store/**'],
+      include: ['engine/**', 'store/**'],
     },
   },
 });