Add Playwright e2e for the download detail Activity tab

[gennitdev]

Summary

There is no end-to-end (Playwright) coverage for the download detail Activity tab. The only download-detail e2e, tests/playwright/mocked/downloads/downloadLabels.spec.ts, is .skipped because the route requires extensive GraphQL mocking.

What exists today

The Activity tab and its pieces are well covered by unit tests (added in #184):

• DownloadTabNavigation.spec.ts (3 tabs + routing + active state)
• activity.spec.ts (empty state, title history, label history, both)
• LabelChangeHistory.spec.ts (actor/badge/collapse)
• DiscussionTitleVersions.spec.ts (old→new, author, show-older)
• description.spec.ts (no title history on Description tab; ErrorBanner)

What's missing

A browser-level test that the tabs navigate and the Activity tab renders label-change + title-edit history end-to-end. This requires mocking the download detail route's ~8 operations: getDiscussion (full shape incl. PastTitleVersions and DiscussionChannels[].LabelChangeHistory), getDiscussionComments, getDiscussionChannelCommentAggregate, getDiscussionChannelRootCommentAggregate, getChannel, getServerConfig, getPublicCollectionsForDownload, plus base handlers.

Acceptance

• Un-skip / replace downloads/downloadLabels.spec.ts with a passing mocked e2e that:
  • loads the download detail page with the 3 tabs (Description / Comments / Activity),
  • clicks Activity and asserts label-change and title-edit entries render,
  • covers the empty-activity case.

Context

Deferred in #184 (consistent with the existing .skip rationale "functionality covered by unit tests").