Mock HTMLElement.scrollTo in test setup

jsdom does not implement scrollTo on elements, causing any component that
calls element.scrollTo() (e.g. MiniDraftGrid's scroll-to-current-cell
effect) to throw in unit tests.

https://claude.ai/code/session_017JCShLVs9xZ6FZmyrUFaE1

app/test/setup.ts

@@ -10,6 +10,9 @@ afterEach(() => {
 // Mock environment variables
 process.env.NODE_ENV = 'test';
 
+// jsdom does not implement scrollTo on elements
+HTMLElement.prototype.scrollTo = vi.fn();
+
 // Mock BetterAuth
 vi.mock('~/lib/auth.server', () => ({
   auth: {