feat: file manager with browse, search, upload, chmod/chown

- Full file browser page at /files with lazy-load, breadcrumbs, drag&drop
- FileBrowserModal component for path selection from Samba/NFS forms
- PathField component replaces bare inputs in share/export forms
- Backend: /api/files/* routes with List, Mkdir, Rename, Delete, Chmod, Chown, Upload, Download, Preview, Search
- Reuses NASCTL_ALLOWED_ROOTS for path validation
- NASCTL_UPLOAD_MAX_BYTES (100MB) and NASCTL_PREVIEW_MAX_BYTES (256KB) env vars
- Capabilities endpoint returns chmod/chown availability (requires root)
- Version bump: 0.3.2 -> 0.4.0

web/src/App.tsx

@@ -10,6 +10,7 @@ import Samba from "./pages/Samba";
 import Nfs from "./pages/Nfs";
 import Log from "./pages/Log";
 import Settings from "./pages/Settings";
+import Files from "./pages/Files";
 
 type AuthState = { loading: boolean; authenticated: boolean; username: string };
 
@@ -52,6 +53,7 @@ export default function App() {
         >
           <Route path="/" element={<Dashboard />} />
           <Route path="/users" element={<Users />} />
+          <Route path="/files" element={<Files />} />
           <Route path="/samba" element={<Samba />} />
           <Route path="/nfs" element={<Nfs />} />
           <Route path="/log" element={<Log />} />