Add session validation UI and expose isValidating in auth context

- LoginPage now shows a loading spinner while validating the session
- Redirect to dashboard automatically once validation completes and session is valid
- Expose isValidating state through AuthProvider for components to track validation status
- Update useAuth hook to return isValidating along with isAuthenticated

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

frontend/src/providers/AuthProvider.tsx

@@ -62,6 +62,8 @@ import { useSessionValidation } from "../hooks/useSessionValidation";
 export interface AuthContextValue {
   /** `true` when the backend considers the session valid. */
   isAuthenticated: boolean;
+  /** `true` while the session is being validated on app mount. */
+  isValidating: boolean;
   /**
    * Authenticate with the master password.
    * Throws an `ApiError` on failure.
@@ -216,8 +218,8 @@ export function AuthProvider({
   }, []);
 
   const value = useMemo<AuthContextValue>(
-    () => ({ isAuthenticated, login, logout }),
-    [isAuthenticated, login, logout],
+    () => ({ isAuthenticated, isValidating, login, logout }),
+    [isAuthenticated, isValidating, login, logout],
   );
 
   // Show loading spinner while validating session on mount.