fix: P1-02 OAuth context propagation and P1-16 AuthProvider double-check

P1-02: OAuth ExchangeCode and GetUserInfo now accept context parameter
       to properly propagate request context to HTTP calls
P1-16: AuthProvider isAuthenticated now uses single source of truth
       (effectiveUser !== null) instead of double-checking both
       React state and module-level function

frontend/admin/src/app/providers/AuthProvider.tsx

@@ -186,7 +186,7 @@ export function AuthProvider({ children }: AuthProviderProps) {
     user: effectiveUser,
     roles: effectiveRoles,
     isAdmin,
-    isAuthenticated: effectiveUser !== null && isAuthenticated(),
+    isAuthenticated: effectiveUser !== null,
     isLoading,
     onLoginSuccess,
     logout,