fix: restore broken tests after prerelease workflow refactor

- Export __clearVersionCacheForTests() from adminService; call in
  versionNotification beforeEach to reset module-scoped cache between
  tests (VNOTIF-002..006 failed because VNOTIF-001 cached
  update_available:false, short-circuiting all subsequent test fetches)
- Seed appVersion:'2.9.10' in Navbar test authStore; appVersion moved
  from local useEffect state to authStore in last commit so the test
  render no longer fetches it independently (FE-COMP-NAVBAR-016)
- Add data-testid="weekend-days" to VacaySettings weekend-days
  container; use within() in tests to scope button count to that
  section, fixing false positives from the week-start buttons which
  share the same inline styles (FE-COMP-VACAYSETTINGS-003/004)
- Pass isPrerelease={true} in GitHubPanel FE-ADMIN-GH-007; component
  filters out prerelease releases when isPrerelease=false so the badge
  was never rendered (pre-existing, unrelated to last commit)

client/src/components/Admin/GitHubPanel.test.tsx

@@ -133,7 +133,7 @@ describe('GitHubPanel', () => {
     server.use(
       http.get('/api/admin/github-releases', () => HttpResponse.json([r])),
     );
-    render(<GitHubPanel />);
+    render(<GitHubPanel isPrerelease={true} />);
     await screen.findByText('v3.0.0-beta.1');
     expect(screen.getByText('Pre-release')).toBeInTheDocument();
   });