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.

先阅读 PR #28 和现有的公开屏幕 smoke tests，然后检查 apps/web/tests/test-utils.tsx 中的 renderScreen()、mockOk() 和 mockPaginated()。为 PersonDetail、TagDetail、两个 feed、Volunteer 和 Sponsor 添加覆盖，并断言所列出的 Display Rules；完成的标准是每个屏幕都有两或三个 smoke-test 断言，并且测试套件通过。

由索引模型根据 Issue 内容生成。