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>

README.md

@@ -7,7 +7,7 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
 > Life) + Phase 4 (Operations) + Phase 5 (Multi-host) + Phase 6 (Backup
 > destinations) + Phase 7 (Scheduled backups) + Phase 8 (Remote-stack backups)
 > + Phase 9 (Networks) + Phase 10 (iGPU passthrough) + Phase 11 (Remote UX &
-> network attach) complete.
+> network attach) + Phase 12 (File browser) complete.
 
 ## What works today (Phase 1)
 
@@ -145,6 +145,23 @@ as intuitive as Dockge, as capable as Portainer for Compose workflows.
   container or connect any container on the host (`POST /api/networks/{id}/connect`
   / `/disconnect`).
 
+### Phase 12 — File browser
+
+- **Files page (sidebar)**: a full host filesystem browser with breadcrumb
+  navigation, clickable browse-root chips, an *Up* control, and a show/hide
+  hidden-files toggle. Listings show size, permissions and modified time.
+- **View & edit**: clicking a text file opens it in a Monaco editor (with syntax
+  highlighting picked from the extension). Binary and oversized files are
+  detected and offered as a download instead. Admins can edit and **Save**.
+- **Manage** (admin): create folders/files, rename, delete (recursive for
+  folders), upload files, and download any file. Every mutation is audit-logged.
+- **Sandboxed**: all access is confined to `ALLOWED_BROWSE_ROOTS`; path
+  traversal and deleting a browse root are refused. To reach the real host
+  filesystem, mount it into the backend and set `HOST_ROOT_PREFIX` (see the
+  commented `/:/host_root` volume in `docker-compose.yml`). Endpoints live under
+  `/api/files/*` (`list`, `read`, `write`, `mkdir`, `touch`, `rename`, `upload`,
+  `download`, `DELETE`).
+
 ## Deploying an agent on another host
 
 ```bash