feat(community): add Discover browse-and-join UI

[lilyshen0722]

Summary

• split the Community sidebar into membership-backed Joined and lazy-loaded Discover views
• list joinable non-member pods with description/member count and one-click join; move successful joins into Joined, refresh memberships, and navigate into the pod
• keep discovery data isolated from All/Team/Private and show specific invite-required / rate-limit guidance
• add EN + zh-CN chrome (Discover/发现, Join/加入) and load-bearing narrow-sidebar layout guards

Verification

• cd frontend && npm test -- --watchAll=false --runInBand — 63 suites, 284 tests passed
• focused sidebar + layout + translation guard — 3 suites, 30 tests passed
• cd frontend && npm run build — passed
• changed-file ESLint — 0 errors (repository-standard TSX extension warnings only)
• real browser: EN + zh-CN at 1280×900 and 390×844; document/sidebar/list overflow all 0 in all four cells

Review notes

• Discover is fetched only after the sub-tab is opened; it never enters the personal pods dataset.
• Client-side personal-pod exclusion is retained as defense in depth even though scope=discover already excludes DM types.
• zh-CN strings follow the locked glossary (社区 / 发现 / 加入, Pod untranslated) and are ready for Sam’s native-language pass.

Closes #727