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/components/FormulaSuggest.vue

@@ -0,0 +1,71 @@
+<script setup lang="ts">
+/**
+ * Generic name/function autocomplete dropdown -- used both for a formula
+ * editor's function-name autocomplete (see useFormulaAutocomplete.ts) and
+ * the Name Box's table/sheet/named-range autocomplete (see document.ts's
+ * `nameBoxSuggestions()`), anything shaped like {name, hint}. Purely
+ * presentational -- positions itself just under whatever it's rendered
+ * inside of (that parent needs `position: relative`); no caret-pixel
+ * tracking.
+ */
+defineProps<{ items: { name: string; hint: string }[]; activeIndex: number }>();
+const emit = defineEmits<{ (e: 'pick', index: number): void }>();
+</script>
+
+<template>
+  <ul class="formula-suggest" role="listbox">
+    <li
+      v-for="(item, i) in items"
+      :key="item.name"
+      role="option"
+      :aria-selected="i === activeIndex"
+      :class="{ on: i === activeIndex }"
+      @mousedown.prevent="emit('pick', i)"
+    >
+      <span class="name">{{ item.name }}</span>
+      <span class="hint">{{ item.hint }}</span>
+    </li>
+  </ul>
+</template>
+
+<style scoped>
+.formula-suggest {
+  position: absolute;
+  top: 100%;
+  left: 0;
+  z-index: 30;
+  margin: 2px 0 0;
+  padding: 4px;
+  min-width: 220px;
+  max-width: 420px;
+  max-height: 220px;
+  overflow-y: auto;
+  list-style: none;
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-radius: var(--radius-sm);
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.15);
+  font-size: 12px;
+}
+.formula-suggest li {
+  display: flex;
+  align-items: baseline;
+  gap: 8px;
+  padding: 5px 8px;
+  border-radius: 5px;
+  cursor: pointer;
+  white-space: nowrap;
+}
+.formula-suggest li.on {
+  background: var(--accent-weak);
+}
+.name {
+  font-weight: 600;
+  color: var(--text);
+}
+.hint {
+  color: var(--text-dim);
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+</style>