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/io/num.ts

@@ -0,0 +1,62 @@
+/**
+ * The native `.num` document file: a ZIP container around the same JSON the
+ * `.json` export writes (json.ts), so it's smaller on disk and leaves room
+ * for more parts later (images, attachments) without changing the format.
+ *
+ * Layout, modelled on OpenDocument:
+ *   mimetype        "application/x-numolus", stored uncompressed as the
+ *                   first entry, so a file sniffer can identify the format
+ *                   from a fixed offset
+ *   meta.json       { format, formatVersion, created (ISO 8601) }
+ *   document.json   the document, exactly as `serializeDocument()` writes it
+ *
+ * Reading only needs `document.json`; `mimetype` is checked when present so
+ * a random ZIP renamed to `.num` fails with a clear message.
+ */
+import { strFromU8, strToU8, unzipSync, zipSync } from 'fflate';
+
+export const NUM_MIMETYPE = 'application/x-numolus';
+export const NUM_FORMAT_VERSION = 1;
+
+export class NumFileError extends Error {}
+
+export function packNum(documentJson: string, now: Date = new Date()): Uint8Array {
+  const meta = { format: 'numolus', formatVersion: NUM_FORMAT_VERSION, created: now.toISOString() };
+  return zipSync({
+    mimetype: [strToU8(NUM_MIMETYPE), { level: 0 }],
+    'meta.json': strToU8(JSON.stringify(meta, null, 2)),
+    'document.json': strToU8(documentJson),
+  });
+}
+
+/** The `document.json` text inside a `.num` file. */
+export function unpackNum(bytes: Uint8Array): string {
+  let files: Record<string, Uint8Array>;
+  try {
+    files = unzipSync(bytes);
+  } catch {
+    throw new NumFileError('Keine gültige .num-Datei (kein ZIP-Archiv).');
+  }
+  const mimetype = files.mimetype ? strFromU8(files.mimetype).trim() : null;
+  if (mimetype !== null && mimetype !== NUM_MIMETYPE) {
+    throw new NumFileError(`Keine numolus-Datei (Typ "${mimetype}").`);
+  }
+  const doc = files['document.json'];
+  if (!doc) throw new NumFileError('Die .num-Datei enthält kein document.json.');
+  const meta = files['meta.json'] ? safeJson(strFromU8(files['meta.json'])) : null;
+  const version = meta && typeof meta === 'object' ? (meta as { formatVersion?: unknown }).formatVersion : undefined;
+  if (typeof version === 'number' && version > NUM_FORMAT_VERSION) {
+    throw new NumFileError(
+      `Die Datei stammt aus einer neueren numolus-Version (Format ${version}); bitte die App aktualisieren.`,
+    );
+  }
+  return strFromU8(doc);
+}
+
+function safeJson(text: string): unknown {
+  try {
+    return JSON.parse(text);
+  } catch {
+    return null;
+  }
+}