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/views/spa/show.html.erb

@@ -0,0 +1,59 @@
+<!doctype html>
+<html lang="de">
+  <head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+    <meta name="color-scheme" content="light dark">
+    <meta name="theme-color" content="#f5f5f7" media="(prefers-color-scheme: light)">
+    <meta name="theme-color" content="#1c1c1e" media="(prefers-color-scheme: dark)">
+
+    <meta
+      name="description"
+      content="Numolus – browserbasierte Tabellenkalkulation nach dem Vorbild von Apple Numbers, mit selbst geschriebener Formel-Engine. wicom, Vaduz (FL)."
+    >
+    <meta name="author" content="Karl-Heinz Wild">
+    <meta name="publisher" content="wicom – Webentwicklung & digitale Lösungen">
+    <meta name="copyright" content="© wicom, Birkenweg 6, FL-9490 Vaduz – https://wicom.li">
+    <meta name="generator" content="Vue 3 + Vite + Rails">
+    <%= csrf_meta_tags %>
+    <meta name="robots" content="index, follow">
+    <meta name="contact" content="office@wicom.li">
+    <link rel="author" href="https://wicom.li">
+    <link rel="publisher" href="https://wicom.li">
+    <link rel="canonical" href="https://wicom.li/">
+
+    <!-- Open Graph -->
+    <meta property="og:type" content="website">
+    <meta property="og:site_name" content="wicom – Webentwicklung & digitale Lösungen">
+    <meta property="og:locale" content="de_LI">
+    <meta property="og:title" content="Numolus · Tabellenkalkulation">
+    <meta
+      property="og:description"
+      content="Tabellenkalkulation à la Apple Numbers mit eigener Formel-Engine – von wicom, Vaduz (FL)."
+    >
+    <meta property="og:url" content="https://wicom.li/">
+    <meta property="og:image" content="https://wicom.li/numolus-logo.svg">
+
+    <!-- Twitter / X Card -->
+    <meta name="twitter:card" content="summary">
+    <meta name="twitter:title" content="Numolus · Tabellenkalkulation">
+    <meta
+      name="twitter:description"
+      content="Tabellenkalkulation à la Apple Numbers mit eigener Formel-Engine – wicom (FL)."
+    >
+    <meta name="twitter:image" content="https://wicom.li/numolus-logo.svg">
+
+    <link
+      rel="icon"
+      type="image/svg+xml"
+      href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 32 32'%3E%3Crect x='0' y='0' width='20' height='20' fill='%23FFD400'/%3E%3Crect x='6' y='6' width='20' height='20' fill='%230057FF'/%3E%3Crect x='12' y='12' width='20' height='20' fill='%23E8384F'/%3E%3C/svg%3E"
+    >
+    <title>Numolus · Tabellenkalkulation — wicom</title>
+  </head>
+  <body>
+    <div id="app"></div>
+    <script id="numolus-boot" type="application/json"><%= raw json_escape(@boot.to_json) %></script>
+    <%= vite_client_tag %>
+    <%= vite_typescript_tag "application" %>
+  </body>
+</html>