Phase 12: file browser (0.12.0)

Add a full host filesystem browser reachable from the sidebar (/files):
breadcrumb navigation, browse-root chips, show-hidden toggle, and a table
with size/permissions/mtime. Text files open in a Monaco editor (language by
extension); binary/oversized files fall back to download. Admins can create
folders/files, rename, delete (recursive for dirs), upload, and save edits;
download is available to all users. Every mutation is audit-logged.

Backend: new services/file_service.py reuses device_service's sandbox helpers
(confined to ALLOWED_BROWSE_ROOTS, mapped via HOST_ROOT_PREFIX) and rejects
path traversal and deleting a browse root. routers/files.py exposes
/api/files/{list,read,download,write,mkdir,touch,rename,upload,DELETE}
(reads: any user; mutations: admin). device_service.browse entries gained
mtime + symlink (non-breaking).

Deployment: ALLOWED_BROWSE_ROOTS + HOST_ROOT_PREFIX are now env-wired in
docker-compose.yml and .env.example, with a commented /:/host_root mount to
browse/manage the real host filesystem.

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

backend/services/device_service.py

@@ -138,6 +138,8 @@ def browse(path: str = "/", show_hidden: bool = False) -> dict:
                     "type": "dir" if is_dir else "file",
                     "size": st.st_size if not is_dir else None,
                     "permissions": oct(st.st_mode & 0o777),
+                    "mtime": st.st_mtime,
+                    "symlink": os.path.islink(full_real),
                 }
             )
         except OSError: