fix(auth): fix mock responses in tests and block open redirect in login

- Add response object to mockSuccess() in login and signup tests so
  response.headers.get() no longer throws
- Validate ?redirect= param: must start with / and not // to prevent
  redirecting users to external domains

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/(public)/login/page.server.test.ts

@@ -31,8 +31,16 @@ describe('login form action', () => {
 		} as any;
 	}
 
+	function mockSuccess() {
+		return {
+			data: { data: { id: '123' } },
+			error: undefined,
+			response: { headers: { get: vi.fn().mockReturnValue(null) } }
+		};
+	}
+
 	it('calls POST /v1/auth/login with form data', async () => {
-		mockPost.mockResolvedValue({ data: { data: { id: '123' } }, error: undefined });
+		mockPost.mockResolvedValue(mockSuccess());
 
 		try {
 			await actions.default(createEvent({
@@ -52,7 +60,7 @@ describe('login form action', () => {
 	});
 
 	it('redirects to /planner on success by default', async () => {
-		mockPost.mockResolvedValue({ data: { data: { id: '123' } }, error: undefined });
+		mockPost.mockResolvedValue(mockSuccess());
 
 		try {
 			await actions.default(createEvent({
@@ -67,7 +75,7 @@ describe('login form action', () => {
 	});
 
 	it('redirects to ?redirect param when present', async () => {
-		mockPost.mockResolvedValue({ data: { data: { id: '123' } }, error: undefined });
+		mockPost.mockResolvedValue(mockSuccess());
 
 		try {
 			await actions.default(createEvent(
@@ -81,6 +89,36 @@ describe('login form action', () => {
 		}
 	});
 
+	it('falls back to /planner when ?redirect= is an absolute URL', async () => {
+		mockPost.mockResolvedValue(mockSuccess());
+
+		try {
+			await actions.default(createEvent(
+				{ email: 'sarah@example.com', password: 'password123' },
+				'?redirect=https%3A%2F%2Fevil.com'
+			));
+			expect.unreachable();
+		} catch (e: any) {
+			expect(e.status).toBe(303);
+			expect(e.location).toBe('/planner');
+		}
+	});
+
+	it('falls back to /planner when ?redirect= is a protocol-relative URL', async () => {
+		mockPost.mockResolvedValue(mockSuccess());
+
+		try {
+			await actions.default(createEvent(
+				{ email: 'sarah@example.com', password: 'password123' },
+				'?redirect=%2F%2Fevil.com'
+			));
+			expect.unreachable();
+		} catch (e: any) {
+			expect(e.status).toBe(303);
+			expect(e.location).toBe('/planner');
+		}
+	});
+
 	it('rejects empty email with validation error', async () => {
 		const result = await actions.default(createEvent({
 			email: '',