Unit test all six UI widgets

Group all tests under a single `group('PeerMentorDetailScreen UI Widgets', ...)` with nested groups per widget. For image-loading tests in PeerMentorProfileHeader, register a fake `HttpClientRequest` or use the `FakeHttpClient` approach so network image tests are hermetic. For the CertificationAlertBanner countdown, inject a `DateTime now` parameter or use a `Clock` abstraction so tests can set a fixed reference date without `DateTime.now()`. Avoid snapshot-style string matching on rendered text; prefer `find.text(...)` with exact strings derived from the same constants the widgets use, to prevent brittle tests.

Ensure each test file imports only from the `lib/` barrel — never from other test files.

Exclusively widget tests using flutter_test. Organize tests in one file per widget or a single grouped file (group() per widget). Use `testWidgets` for all cases. Inject dependencies via constructor parameters or InheritedWidget/Provider fakes — do not rely on global singletons.

Use `StreamController` fakes for OrgLabelsProvider stream tests. Use `FakeAsync` / `tester.pump(Duration(...))` for countdown timer assertions. After all tests pass, run `flutter test --coverage` and generate lcov report; assert total branch coverage ≥80% in CI via `lcov --summary`.