Consolidate frontend storage keys into constants module

- Move magic strings from AuthProvider, MainLayout, and ThemeProvider to
  frontend/src/utils/constants.ts
- Add STORAGE_KEY_AUTHENTICATED, STORAGE_KEY_SIDEBAR_COLLAPSED, and
  STORAGE_KEY_THEME constants with JSDoc descriptions
- Update all three files to import and use centralized keys
- Prevents key drift and typo regressions across the frontend

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/providers/AuthProvider.tsx

@@ -44,6 +44,7 @@ import { useNavigate } from "react-router-dom";
 import * as authApi from "../api/auth";
 import { setUnauthorizedHandler } from "../api/client";
 import { setAuthErrorHandler } from "../utils/fetchError";
+import { STORAGE_KEY_AUTHENTICATED } from "../utils/constants";
 import { SessionValidationLoading } from "../components/SessionValidationLoading";
 import { useSessionValidation } from "../hooks/useSessionValidation";
 
@@ -65,8 +66,6 @@ export interface AuthContextValue {
 
 export const AuthContext = createContext<AuthContextValue | null>(null);
 
-const IS_AUTHENTICATED_KEY = "bangui_authenticated";
-
 // ---------------------------------------------------------------------------
 // Provider
 // ---------------------------------------------------------------------------
@@ -87,14 +86,14 @@ export function AuthProvider({
   children: React.ReactNode;
 }): React.JSX.Element {
   const [isAuthenticated, setIsAuthenticated] = useState<boolean>(() => {
-    const stored = sessionStorage.getItem(IS_AUTHENTICATED_KEY);
+    const stored = sessionStorage.getItem(STORAGE_KEY_AUTHENTICATED);
     return stored === "true";
   });
   const [isValidating, setIsValidating] = useState<boolean>(isAuthenticated);
   const navigate = useNavigate();
 
   const handleSessionExpired = useCallback((): void => {
-    sessionStorage.removeItem(IS_AUTHENTICATED_KEY);
+    sessionStorage.removeItem(STORAGE_KEY_AUTHENTICATED);
     setIsAuthenticated(false);
     navigate("/login", { replace: true });
   }, [navigate]);
@@ -136,7 +135,7 @@ export function AuthProvider({
 
   const login = useCallback(async (password: string): Promise<void> => {
     await authApi.login(password);
-    sessionStorage.setItem(IS_AUTHENTICATED_KEY, "true");
+    sessionStorage.setItem(STORAGE_KEY_AUTHENTICATED, "true");
     setIsAuthenticated(true);
   }, []);
 
@@ -145,7 +144,7 @@ export function AuthProvider({
       await authApi.logout();
     } finally {
       // Always clear local state even if the API call fails (e.g. expired session).
-      sessionStorage.removeItem(IS_AUTHENTICATED_KEY);
+      sessionStorage.removeItem(STORAGE_KEY_AUTHENTICATED);
       setIsAuthenticated(false);
     }
   }, []);