Add user management (CRUD + admin toggle), make Settings available to all users

- New /admin/users page: create/delete users, toggle admin role, guarded against
  self-demotion/self-deletion
- Backend: GET/POST/PATCH/DELETE /api/admin/users, admin-only
- Settings moved from /admin/settings to /settings (all logged-in users); the
  password-change section shows for everyone, integration/API-key sections only
  render (and only fetch) for admins, since those endpoints stay admin-gated
- Navbar: Settings link now visible to all users; added Users nav link for admins

frontend/src/App.js

@@ -24,6 +24,7 @@ import ShowDetail from "./pages/ShowDetail";
 import EpisodePlayer from "./pages/EpisodePlayer";
 import SonarrImport from "./pages/SonarrImport";
 import AdminShowEpisodes from "./pages/AdminShowEpisodes";
+import Users from "./pages/Users";
 
 const ProfileGate = ({ children }) => {
   const { user, loading: authLoading } = useAuth();
@@ -89,9 +90,10 @@ function App() {
                 <Route path="/search" element={<ProfileGate><Search /></ProfileGate>} />
                 <Route path="/watch/:id" element={<ProfileGate><Player /></ProfileGate>} />
                 <Route path="/requests" element={<ProfileGate><Requests /></ProfileGate>} />
+                <Route path="/settings" element={<ProfileGate><Settings /></ProfileGate>} />
                 <Route path="/admin" element={<AdminGate><Admin /></AdminGate>} />
+                <Route path="/admin/users" element={<AdminGate><Users /></AdminGate>} />
                 <Route path="/admin/upload" element={<AdminGate><AdminUpload /></AdminGate>} />
-                <Route path="/admin/settings" element={<AdminGate><Settings /></AdminGate>} />
                 <Route path="/admin/radarr" element={<AdminGate><RadarrImport /></AdminGate>} />
                 <Route path="/admin/sonarr" element={<AdminGate><SonarrImport /></AdminGate>} />
                 <Route path="/admin/shows/:id/episodes" element={<AdminGate><AdminShowEpisodes /></AdminGate>} />