M4: admin settings page and JSON API

- AdminApiController (admin only, CSRF-protected): status, create key,
  download recovery kit, confirm (statement accepted + exact phrase),
  backends with their usable auth mechanisms, add/test/remove locations,
  start backups, schedule, snapshots, browse, queue restores
- RestoreJob runs web-initiated restores in the background; backup and
  restore runs are kept apart in the run table
- admin section 'NG Backup' with a Vue 3 / @nextcloud/vue 9 page: key and
  recovery-kit flow (download, tick statement, 'Yes, I confirm'), location
  form built from files_external's parameter definitions, schedule, run
  list with polling, snapshot browser and restore with the three modes
- auth mechanisms that depend on a user session or on files_external's
  own stored credentials are not offered

API tested with curl on NC34 (status, backends, schedule validation,
confirmation refusal, browse, queued restore and backup completed by
cron); non-admins get 403, anonymous 401.

src/format.js

@@ -0,0 +1,15 @@
+// SPDX-FileCopyrightText: 2026 Dillard Blom
+// SPDX-License-Identifier: AGPL-3.0-or-later
+export function formatSize(bytes) {
+	if (bytes === null || bytes === undefined) {
+		return ''
+	}
+	const units = ['B', 'KiB', 'MiB', 'GiB', 'TiB']
+	let i = 0
+	let b = bytes
+	while (b >= 1024 && i < units.length - 1) {
+		b /= 1024
+		i++
+	}
+	return i === 0 ? `${b} B` : `${b.toFixed(1)} ${units[i]}`
+}