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/TextBox.vue

@@ -0,0 +1,92 @@
+<script setup lang="ts">
+/**
+ * A formatted text box object. Formatting (size, bold, italic, alignment,
+ * text/background colour) lives on the object and is edited in the inspector;
+ * double-click here edits the text itself.
+ */
+import { computed, nextTick, ref } from 'vue';
+import { useDocument } from '../store/document';
+
+const props = defineProps<{ objId: string }>();
+const store = useDocument();
+
+const obj = computed(() => {
+  const o = store.objectById(props.objId);
+  return o && o.kind === 'text' ? o : undefined;
+});
+
+const editing = ref(false);
+const draft = ref('');
+const area = ref<HTMLTextAreaElement | null>(null);
+
+function beginEdit() {
+  if (!obj.value) return;
+  draft.value = obj.value.text;
+  editing.value = true;
+  nextTick(() => area.value?.focus());
+}
+function commit() {
+  if (obj.value) store.setTextContent(obj.value.id, draft.value);
+  editing.value = false;
+}
+function cancel() {
+  editing.value = false;
+}
+
+const boxStyle = computed(() => {
+  const o = obj.value;
+  if (!o) return {};
+  return {
+    fontSize: o.fontSize + 'px',
+    fontWeight: o.bold ? '600' : '400',
+    fontStyle: o.italic ? 'italic' : 'normal',
+    textAlign: o.align,
+    color: o.color || 'var(--text)',
+    background: o.bg || 'transparent',
+  } as Record<string, string>;
+});
+</script>
+
+<template>
+  <div v-if="obj" class="textbox" :style="boxStyle" @dblclick="beginEdit">
+    <textarea
+      v-if="editing"
+      ref="area"
+      v-model="draft"
+      class="edit"
+      spellcheck="false"
+      @blur="commit"
+      @keydown.esc.prevent="cancel"
+      @keydown.enter.exact.stop
+      @mousedown.stop
+    />
+    <div v-else class="view">{{ obj.text || 'Doppelklick zum Bearbeiten' }}</div>
+  </div>
+</template>
+
+<style scoped>
+.textbox {
+  width: 100%;
+  height: 100%;
+  padding: 8px 10px;
+  overflow: auto;
+  line-height: 1.35;
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+.view {
+  min-height: 100%;
+}
+.edit {
+  width: 100%;
+  height: 100%;
+  border: 0;
+  outline: none;
+  resize: none;
+  padding: 0;
+  font: inherit;
+  color: inherit;
+  text-align: inherit;
+  background: transparent;
+}
+</style>