Replace the simple cycleTheme() with a modal dialog that shows all
19 themes with color swatches, live preview on Up/Down navigation,
and commit/revert on Enter/Esc.
Changes:
- Add modalThemeSelect kind to modalKind enum
- Add themeSelectorState struct + field to Model
- Add openThemeSelector() replacing cycleTheme() call
- Add handleModalKey case for modalThemeSelect (Up/Down/Enter/Esc)
- Add applyThemePreview() and finalizeTheme() helper methods
- Add renderThemeSelectModal() with aligned theme names + swatches
- Update renderModal() dispatch and widen modal for theme selector
- Update help text: 't' now shows 'select theme'
