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)

server/src/services/adminService.ts

@@ -328,6 +328,11 @@ interface VersionInfo {
 const VERSION_CACHE_TTL = 5 * 60 * 1000; // 5 minutes
 let _versionCache: { data: VersionInfo; expiresAt: number } | null = null;
 
+/** Test-only: clear the in-memory version cache. */
+export function __clearVersionCacheForTests(): void {
+  _versionCache = null;
+}
+
 export async function checkVersion(): Promise<VersionInfo> {
   if (_versionCache && Date.now() < _versionCache.expiresAt) {
     return _versionCache.data;