Add keyboard-driven date navigation to the TUI:
- [ and ] keys select previous/next day, adjusting the visible range
- g opens a date entry modal for direct date navigation
- t returns to today's live view from any historical browsing
- Background refresh preserves the browsed selection
- Local-day widget shows data for the selected date
- Updated help screen and action legend with new bindings
- DatePickerScreen modal with input validation
- 24 headless tests covering navigation, date entry, browse
stability, local-day evidence, and constrained widths
Co-authored-by: CommandCodeBot <noreply@commandcode.ai>
