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/useFormulaAutocomplete.ts

@@ -0,0 +1,49 @@
+/**
+ * Function-name autocomplete for a formula editor (see FormulaBar.vue and
+ * TableGrid.vue's in-cell editor, which both wire this in the same way).
+ *
+ * Unlike useTheme.ts/useToasts.ts (module-level singletons), this factory
+ * creates fresh, independent state per call -- each formula editor gets its
+ * own dropdown/selection, since two editors are never showing suggestions
+ * for the same formula at once.
+ */
+import { ref } from 'vue';
+import { currentIdentifierAtCaret } from '../engine';
+import { useDocument } from '../store/document';
+import type { FnSuggestion } from '../store/types';
+
+export function useFormulaAutocomplete() {
+  const store = useDocument();
+  const suggestions = ref<FnSuggestion[]>([]);
+  const activeIndex = ref(0);
+
+  /** Recompute the dropdown for the current buffer + caret -- call this
+   *  after every edit to the text (an `input` handler). */
+  function update(text: string, caret: number) {
+    const hit = currentIdentifierAtCaret(text, caret);
+    suggestions.value = hit && hit.text ? store.functionSuggestions(hit.text) : [];
+    activeIndex.value = 0;
+  }
+  function close() {
+    suggestions.value = [];
+  }
+  /** Move the highlighted suggestion by `delta` (wraps). No-op while closed. */
+  function move(delta: number) {
+    const n = suggestions.value.length;
+    if (!n) return;
+    activeIndex.value = (activeIndex.value + delta + n) % n;
+  }
+  /** The replacement the caller should apply via its own `replace(start, end,
+   *  value)` (the same one point-mode already uses) to accept the currently
+   *  highlighted suggestion -- `null` when no dropdown is open, so the
+   *  caller falls through to its normal Tab/Enter handling. Appends "(" ,
+   *  ready for arguments, same as Excel/Sheets' own autocomplete. */
+  function accept(text: string, caret: number): { start: number; end: number; value: string } | null {
+    if (!suggestions.value.length) return null;
+    const hit = currentIdentifierAtCaret(text, caret);
+    if (!hit) return null;
+    return { start: hit.start, end: caret, value: suggestions.value[activeIndex.value].name + '(' };
+  }
+
+  return { suggestions, activeIndex, update, close, move, accept };
+}