Add screen smoke tests for the public-screens detail/feed/static screens

**Context:** PR #28 (`feat/public-screens`) shipped screen smoke tests for Home, ProjectsIndex, ProjectDetail, HelpWantedIndex — the screens with the most spec-bound Display Rules. The remaining screens were exercised only via the build + browser walkthrough, not unit tests.

**Add smoke tests for:**

- [ ] `PersonDetail` — renders fullName + "Member since {month yyyy}" + projects list sorted by maintainer / joinedAt
- [ ] `TagDetail` — renders the three sections (Projects / Help-wanted / Members), hides Members on `event` namespace
- [ ] `ProjectUpdatesFeed` — renders cards reverse-chron, filtered empty state
- [ ] `ProjectBuzzFeed` — same shape, external-link headlines
- [ ] `Volunteer` — hero CTA toggles on auth state, live project count uses /api/projects?perPage=1
- [ ] `Sponsor` — static content + copy-to-clipboard button

Each test: render with `renderScreen()` from `apps/web/tests/test-utils.tsx`, mock `fetch` with `mockOk` / `mockPaginated`, assert two or three load-bearing Display Rules from the spec.

Scoped small enough for a single follow-up PR.