Add a recent posts Home view

[benvinegar]

Summary

• add a Home view for multi-session workspaces backed by /api/posts/recent
• render recent post cards with sandboxed surface previews and deep links to full posts
• keep existing single-session and last-session boot behavior while letting Home own the no-selection multi-session root

Stacked on #219.
Fixes #216

Validation

• npm run typecheck
• npm run lint
• npm run format:check
• npm test
• npm run build
• npx playwright test --project=chromium

This PR description was generated by Pi using GPT-5 Codex Mini