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/UploadModal.vue

@@ -0,0 +1,93 @@
+<script setup lang="ts">
+/**
+ * Shown once after signing in when this browser still has local documents:
+ * which of them should move into the account (server)? Uploaded ones are
+ * removed from the browser. "Später" leaves everything as is -- uploading
+ * stays possible from the document list.
+ */
+import { ref, watch } from 'vue';
+import { useLibrary } from '../store/library';
+import BaseModal from './BaseModal.vue';
+
+const props = defineProps<{ open: boolean }>();
+const emit = defineEmits<{ (e: 'close'): void }>();
+
+const library = useLibrary();
+const selected = ref<Set<string>>(new Set());
+const busy = ref(false);
+
+watch(
+  () => props.open,
+  (o) => {
+    if (o) selected.value = new Set(library.state.docs.map((d) => d.id));
+  },
+);
+
+function toggle(id: string) {
+  const s = new Set(selected.value);
+  if (s.has(id)) s.delete(id);
+  else s.add(id);
+  selected.value = s;
+}
+
+async function run() {
+  busy.value = true;
+  try {
+    await library.upload([...selected.value]);
+    emit('close');
+  } finally {
+    busy.value = false;
+  }
+}
+</script>
+
+<template>
+  <BaseModal :open="open" title="Dokumente in dein Konto übernehmen?" @close="emit('close')">
+    <p class="lead">
+      In diesem Browser liegen noch Dokumente. Ausgewählte werden in dein Konto hochgeladen – dann sind sie
+      auf allen Geräten da und lassen sich teilen – und hier aus dem Browser entfernt.
+    </p>
+    <ul class="list">
+      <li v-for="d in library.state.docs" :key="d.id">
+        <label>
+          <input type="checkbox" :checked="selected.has(d.id)" @change="toggle(d.id)" />
+          {{ d.title }}
+        </label>
+      </li>
+    </ul>
+    <template #footer>
+      <button @click="emit('close')">Später</button>
+      <button class="primary" :disabled="busy || !selected.size" @click="run">
+        {{ busy ? 'Lade hoch …' : `${selected.size} hochladen` }}
+      </button>
+    </template>
+  </BaseModal>
+</template>
+
+<style scoped>
+.lead {
+  margin: 0 0 12px;
+  font-size: 13px;
+  line-height: 1.5;
+}
+.list {
+  list-style: none;
+  margin: 0;
+  padding: 0;
+  max-height: 40vh;
+  overflow-y: auto;
+}
+.list li + li {
+  margin-top: 4px;
+}
+.list label {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 6px 8px;
+  border-radius: 6px;
+  background: var(--surface-2);
+  font-size: 13px;
+  cursor: pointer;
+}
+</style>