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/config/brand.ts

@@ -0,0 +1,37 @@
+/**
+ * Publisher / company metadata.
+ * Source: https://wicom.li  (fetched 2026-08-29).
+ */
+export const WICOM = {
+  name: 'wicom',
+  legalName: 'wicom – Wild Karl-Heinz',
+  descriptor: 'Webentwicklung & digitale Lösungen',
+  tagline: 'Unsere Arbeiten – Ihr Erfolg',
+  since: 1997,
+  url: 'https://wicom.li',
+  email: 'office@wicom.li',
+  phone: '+41 79 446 2901',
+  phoneHref: 'tel:+41794462901',
+  address: {
+    street: 'Birkenweg 6',
+    city: 'FL-9490 Vaduz',
+    country: 'Liechtenstein',
+  },
+  author: 'Karl-Heinz Wild',
+} as const;
+
+/** One-line description of this app, used for `<meta name="description">` / OG. */
+export const APP_DESCRIPTION =
+  'Numolus — browserbasierte Tabellenkalkulation nach dem Vorbild von Apple Numbers, ' +
+  'mit selbst geschriebener Formel-Engine. wicom, Vaduz, FL.';
+
+/**
+ * Social-media profiles of wicom.
+ * wicom.li lists none — add `{ label, url }` entries here and they appear in the
+ * header and the "Über"-Dialog automatically.
+ */
+export const WICOM_SOCIAL: ReadonlyArray<{ label: string; url: string }> = [
+  // { label: 'LinkedIn', url: 'https://www.linkedin.com/company/…' },
+  // { label: 'GitHub',   url: 'https://github.com/…' },
+  // { label: 'X',        url: 'https://x.com/…' },
+];