Add collapsible diff panel in task detail view

[robinnewhouse]

Summary

• add a dedicated collapsed state for the task detail diff panel
• let the agent panel expand when the diff is hidden and add an explicit Diff View restore button
• cover collapse/restore and expanded-mode interactions in card detail view tests

Testing

• npm test -- src/components/card-detail-view.test.tsx
• pre-commit checks: npm run lint && npm run typecheck && npm run test

Screenshots