0.33.0: NFS share as backup destination

New destination type 'nfs' alongside SFTP and S3. The StackPilot container
needs no mount privileges: the Docker daemon mounts the export as a named
volume (stackpilot-nfs-dest-<id>, driver local/type nfs, recreated whenever
server/path/options change) and all file I/O runs through throwaway helper
containers (BACKUP_HELPER_IMAGE) — upload via put_archive, list via stat,
download via get_archive, delete/test via short-lived runs. Config: server,
export path, mount options (default rw), optional subdirectory (sanitized;
shell-safe charset). Mount failures surface as clean destination errors.

Settings UI gains the NFS form + summary; works everywhere destinations are
used (push, restore-from, scheduled backups incl. retention).

Verified live against a real kernel NFS server: test, push (file on the
export), list, restore-from incl. volume data, remote delete, config change
recreates the mount volume, unreachable server fails cleanly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.32.1",
+  "version": "0.33.0",
   "type": "module",
   "scripts": {
     "dev": "vite",