Replace sidebar session/agent counts with navigation icons

## Summary - Replace '2 sessions n agents' text with intuitive switch icons for navigation - Extract reusable SwitchIcon component for consistent navigation patterns - Rename "Active Session" to "Session" for cleaner UI - Fix spacing consistency between project and session cards ## Test plan - [x] Verify switch icons appear next to "Workspace" and "Session" titles - [x] Confirm clicking switch icons navigates to project sessions page - [x] Test mobile navigation behavior (closes mobile nav) - [x] Ensure consistent card heights between project and session sections - [x] All tests pass (removed obsolete session/agent count tests, added switch icon tests) 🤖 Generated with [Claude Code](https://claude.ai/code)