feat: theme selector dialog with live preview (t key)

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'

internal/ui/keymap.go

@@ -53,7 +53,7 @@ func DefaultKeyMap() KeyMap {
 		Info:           key.NewBinding(key.WithKeys("f9", "o"), key.WithHelp("F9/o", "info")),
 		SelectText:     key.NewBinding(key.WithKeys("ctrl+t"), key.WithHelp("C-t", "text select")),
 		ToggleHidden:   key.NewBinding(key.WithKeys("."), key.WithHelp(".", "hidden")),
-		CycleTheme:     key.NewBinding(key.WithKeys("t"), key.WithHelp("t", "theme")),
+		CycleTheme:     key.NewBinding(key.WithKeys("t"), key.WithHelp("t", "select theme")),
 		CycleSort:      key.NewBinding(key.WithKeys("g"), key.WithHelp("g", "sort")),
 		SSH:            key.NewBinding(key.WithKeys("f12", "s"), key.WithHelp("F12/s", "ssh")),
 		Mirror:         key.NewBinding(key.WithKeys("p"), key.WithHelp("p", "mirror pane")),