feat(backup): admin and account UI for server backups, import by section

Admin settings: switch, time, retention, list with download and a
"Jetzt sichern" button. Account settings: own server backups with
download and restore (replace or add, for chosen sections, replace needs
a second tap). The MindSift import gets the same section picker.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/download.ts

@@ -0,0 +1,15 @@
+// Saves a downloaded file. Works on mobile browsers too: the link must be in the document and the
+// object URL alive until the click ran.
+export function saveBlob(blob: Blob, filename: string): void {
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  a.remove();
+  window.setTimeout(() => URL.revokeObjectURL(url), 10000);
+}
+
+export const formatBytes = (n: number): string =>
+  n < 1024 ? `${n} B` : n < 1024 * 1024 ? `${(n / 1024).toFixed(0)} KB` : `${(n / 1024 / 1024).toFixed(1)} MB`;