feat: add View all in folder modal for Resume Conversation (#94)

Drill into a single project's complete history when the homepage's
3-per-project dedup hides older conversations.

- Backend: /api/history/sessions accepts projectKey/offset/limit;
  single-folder mode bypasses the 50-cap and returns { sessions, total }.
  projectKey is validated against ^[A-Za-z0-9_-]+$ to prevent traversal.
- Frontend: detail panel adds "View all in this folder" button that
  opens a modal listing 20 sessions per page with Show more pagination.
- Modal items reuse _buildHistoryItem with showViewAll:false to avoid
  recursive entry points.

Co-authored-by: Teigen <teigen@TeigendeMac-mini.local>