feat(sessions): add collapsible pinned section

[Zhangs-11]

Summary

• add a hover action to pin or unpin a session within the current project
• move pinned session families into a collapsible section without duplicating them in the regular list
• add a card-level setting for the global default collapsed state
• persist pins and the default preference in browser localStorage, and prune stale pins after sessions disappear

Test plan

• npm test (891 passing)
• node_modules/.bin/tsc --noEmit
• npm run lint
• git diff --check
• verified pinning, unpinning, project isolation, collapse/expand, reload persistence, and the global default setting in a real browser

Demo

Demo provenance:

• demonstrated commit: 0da9c81de8d2cdc5d5fe2d782988d2fb82d18c7e
• served from the clean feat/collapsible-pinned-sessions worktree at http://127.0.0.1:30156
• mode: Next.js development server (the repository instructions prohibit next build during development)
• browser: fresh isolated headless Chrome profile; the repository-declared Playwright CDP fallback was used after agent-browser control became unavailable
• transport: normal Pi Web API/RPC flow; no fixtures, mocks, DOM-event injection, or test-only hooks
• real model rounds: yes — two GPT-6-Astra rounds created the demonstrated sessions

Related to #406.

[agegr]

Thanks for the contribution. I plan to redesign session organization more broadly, so I am closing this PR for now.