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/tests/unit/services/versionNotification.test.ts

@@ -34,7 +34,7 @@ import { createTables } from '../../../src/db/schema';
 import { runMigrations } from '../../../src/db/migrations';
 import { resetTestDb } from '../../helpers/test-db';
 import { createAdmin } from '../../helpers/factories';
-import { checkAndNotifyVersion } from '../../../src/services/adminService';
+import { checkAndNotifyVersion, __clearVersionCacheForTests } from '../../../src/services/adminService';
 
 // Helper: mock the GitHub releases/latest endpoint
 function mockGitHubLatest(tagName: string, ok = true): void {
@@ -63,6 +63,7 @@ beforeAll(() => {
 
 beforeEach(() => {
   resetTestDb(testDb);
+  __clearVersionCacheForTests();
   vi.unstubAllGlobals();
 });