feat(auth): preserve redirect URL when redirecting to /login

Appends ?redirect= with the original pathname so the login page
can redirect back after successful authentication.

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

frontend/src/hooks.server.test.ts

@@ -57,15 +57,14 @@ describe('auth guard (hooks.server.ts handle)', () => {
 		}
 	);
 
-	it('redirects unauthenticated requests on protected routes', async () => {
-		const { event, resolve } = createEvent('/planner');
+	it('redirects unauthenticated requests to /login with redirect param', async () => {
+		const { event, resolve } = createEvent('/recipes/abc');
 		try {
 			await handle({ event, resolve });
-			// If using SvelteKit redirect, it throws
 			expect.unreachable();
 		} catch (e: any) {
 			expect(e.status).toBe(302);
-			expect(e.location).toBe('/login');
+			expect(e.location).toBe('/login?redirect=%2Frecipes%2Fabc');
 		}
 	});
 
@@ -99,7 +98,7 @@ describe('auth guard (hooks.server.ts handle)', () => {
 		expect(resolve).toHaveBeenCalledWith(event);
 	});
 
-	it('redirects to /login when session validation fails', async () => {
+	it('redirects to /login with redirect param when session validation fails', async () => {
 		mockGet.mockResolvedValue({ data: undefined, error: { status: 401 } });
 
 		const { event, resolve } = createEvent('/planner', 'bad-session');
@@ -108,7 +107,7 @@ describe('auth guard (hooks.server.ts handle)', () => {
 			expect.unreachable();
 		} catch (e: any) {
 			expect(e.status).toBe(302);
-			expect(e.location).toBe('/login');
+			expect(e.location).toBe('/login?redirect=%2Fplanner');
 		}
 	});
 });