test: fix 5 failing tests after react-day-picker addition (#101)

PR #98 added react-day-picker via shadcn/ui Calendar but the package
was not yet installed in node_modules. This broke 5 test files at import
resolution time:

- src/App.test.tsx
- src/components/Editor.test.tsx
- src/components/Inspector.test.tsx
- src/components/InspectorPanels.test.tsx
- src/components/DynamicPropertiesPanel.test.tsx

Fix: add a vitest mock for react-day-picker in the global test setup
(same pattern as the existing react-virtuoso mock). DayPicker renders
a real calendar widget that requires DOM APIs unavailable in jsdom —
mocking it to null is the correct approach for unit/integration tests
that don't test date-picker behavior specifically.

Before: 5 failed | 48 passed (764 tests)
After:  0 failed | 53 passed (929 tests)

src/test/setup.ts

@@ -10,6 +10,12 @@ vi.mock('@tauri-apps/plugin-opener', () => ({
   openUrl: vi.fn(),
 }))
 
+// Mock react-day-picker: Calendar component uses DayPicker which needs real DOM APIs not available in jsdom
+vi.mock('react-day-picker', () => ({
+  DayPicker: () => null,
+  getDefaultClassNames: () => ({}),
+}))
+
 // Mock react-virtuoso: JSDOM has no real viewport, so render all items directly
 vi.mock('react-virtuoso', () => ({
   Virtuoso: ({ data, itemContent, components }: {