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

[themightychris]

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.