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/composables/useTheme.ts

@@ -0,0 +1,37 @@
+import { ref, watchEffect } from 'vue';
+
+type Mode = 'system' | 'light' | 'dark';
+
+const KEY = 'numolus:theme';
+/** Key used before the rename to numolus -- still read as a fallback. */
+const LEGACY_KEY = 'numbers-study:theme';
+const stored =
+  ((localStorage.getItem(KEY) ?? localStorage.getItem(LEGACY_KEY)) as Mode | null) ??
+  'system';
+const mode = ref<Mode>(stored);
+
+watchEffect(() => {
+  const root = document.documentElement;
+  if (mode.value === 'system') root.removeAttribute('data-theme');
+  else root.setAttribute('data-theme', mode.value);
+  try {
+    localStorage.setItem(KEY, mode.value);
+  } catch {
+    /* ignore */
+  }
+});
+
+/** True when the effective theme is dark (used to recolour Chart.js). */
+function isDarkNow(): boolean {
+  if (mode.value === 'dark') return true;
+  if (mode.value === 'light') return false;
+  return window.matchMedia('(prefers-color-scheme: dark)').matches;
+}
+
+export function useTheme() {
+  function cycle() {
+    mode.value =
+      mode.value === 'system' ? 'light' : mode.value === 'light' ? 'dark' : 'system';
+  }
+  return { mode, cycle, isDarkNow };
+}