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/components/layout/Sidebar.tsx

@@ -4,6 +4,7 @@ import {
   Boxes,
   Network,
   Image,
+  FolderTree,
   LayoutTemplate,
   ScrollText,
   Settings,
@@ -22,6 +23,7 @@ const nav = [
   { to: "/stacks", label: "Stacks", icon: Boxes },
   { to: "/networks", label: "Networks", icon: Network },
   { to: "/images", label: "Images", icon: Image },
+  { to: "/files", label: "Files", icon: FolderTree },
   { to: "/templates", label: "Templates", icon: LayoutTemplate },
   { to: "/audit", label: "Audit log", icon: ScrollText },
   { to: "/settings", label: "Settings", icon: Settings },