test(dropzone): replace setTimeout flake with vi.waitFor + hoisted mock

The "no-callback" and "no-prop" tests no longer rely on an arbitrary
50ms sleep. Test 2 awaits the mocked invalidateAll call (the last async
step of the upload handler) before asserting the callback was not
invoked. Test 3 lets vitest-browser-svelte's own expect.element poll
until the success message appears.

Addresses Sara's and Felix's review concern about flake-prone timing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/DropZone.svelte.spec.ts

@@ -4,7 +4,10 @@ import { page } from 'vitest/browser';
 
 import DropZone from './DropZone.svelte';
 
-vi.mock('$app/navigation', () => ({ invalidateAll: vi.fn(async () => {}) }));
+// vi.hoisted lets the mock fn reference survive vi.mock's hoisting so tests
+// can assert on it from below while the factory remains self-contained.
+const { invalidateAllMock } = vi.hoisted(() => ({ invalidateAllMock: vi.fn(async () => {}) }));
+vi.mock('$app/navigation', () => ({ invalidateAll: invalidateAllMock }));
 
 afterEach(() => {
 	cleanup();
@@ -62,8 +65,11 @@ describe('DropZone onUploadComplete', () => {
 		input.files = dt.files;
 		input.dispatchEvent(new Event('change', { bubbles: true }));
 
-		// Wait a tick to let the microtask flush
+		// invalidateAll is the last async step of the upload handler — once it
-		await new Promise((r) => setTimeout(r, 50));
+		// has been called, the callback decision has already been made.
+		await vi.waitFor(() => {
+			expect(invalidateAllMock).toHaveBeenCalled();
+		});
 		expect(onUploadComplete).not.toHaveBeenCalled();
 	});
 
@@ -75,10 +81,9 @@ describe('DropZone onUploadComplete', () => {
 		const file = new File(['%PDF-1.4'], 'x.pdf', { type: 'application/pdf' });
 		const dt = new DataTransfer();
 		dt.items.add(file);
+		// Should not throw when the optional callback is absent.
 		input.files = dt.files;
-		// Should not throw
 		input.dispatchEvent(new Event('change', { bubbles: true }));
-		await new Promise((r) => setTimeout(r, 50));
 		await expect.element(page.getByText(/1 Dokument/)).toBeInTheDocument();
 	});
 });