File browser: folder upload + copy/move (0.13.0)

Folder upload: the Files page gained an "Upload folder" picker
(webkitdirectory); each file is sent with its webkitRelativePath and the
backend recreates the directory tree. upload_target now accepts an optional
rel_path, creating intermediate dirs (mkdir -p) inside the sandbox with each
component validated against traversal.

Copy/move: new file_service.copy/move + POST /api/files/{copy,move}
(admin, audit-logged). The UI adds per-row copy/cut actions, a clipboard bar
to paste into the current directory, and an overwrite prompt on conflict.
Both refuse to move/copy a folder into itself or its own subtree and are
sandbox-checked on source and destination.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.12.0",
+  "version": "0.13.0",
   "type": "module",
   "scripts": {
     "dev": "vite",