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>

frontend/src/App.tsx

@@ -8,6 +8,7 @@ import { StackDetail } from "@/pages/StackDetail";
 import { StackEditor } from "@/pages/StackEditor";
 import { RemoteStackDetail } from "@/pages/RemoteStackDetail";
 import { Images } from "@/pages/Images";
+import { Files } from "@/pages/Files";
 import { Templates } from "@/pages/Templates";
 import { Settings } from "@/pages/Settings";
 import { Audit } from "@/pages/Audit";
@@ -47,6 +48,7 @@ export default function App() {
             <Route path="/hosts/:agentId/stacks/:id" element={<RemoteStackDetail />} />
             <Route path="/networks" element={<Networks />} />
             <Route path="/images" element={<Images />} />
+            <Route path="/files" element={<Files />} />
             <Route path="/templates" element={<Templates />} />
             <Route path="/audit" element={<Audit />} />
             <Route path="/settings" element={<Settings />} />