fix(auth): read JSESSIONID cookie to match Spring Security default

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

frontend/src/hooks.server.test.ts

@@ -25,7 +25,10 @@ describe('auth guard (hooks.server.ts handle)', () => {
 		const event = {
 			url: new URL(`http://localhost${pathname}`),
 			cookies: {
-				get: vi.fn().mockReturnValue(cookie)
+				get: vi.fn().mockImplementation((name: string) => {
+					if (name === 'JSESSIONID') return cookie;
+					return undefined;
+				})
 			},
 			locals: {} as any,
 			fetch: vi.fn()