test(native): reset the file preview mock between the file opening tests

The second test asserted the preview was not called while the first one's call
was still counted, so it failed whenever both ran in order.

client/src/native/nativeFiles.test.ts

@@ -13,6 +13,7 @@ vi.mock('./trekShell', () => ({ TrekShell: { previewFile: vi.fn() } }));
 beforeEach(() => {
   vi.mocked(Filesystem.writeFile).mockReset().mockResolvedValue({ uri: 'file:///cache/trip.pdf' });
   vi.mocked(Share.share).mockReset().mockResolvedValue({});
+  vi.mocked(TrekShell.previewFile).mockReset();
 });
 
 describe('cacheFileName', () => {