feat(theme): add dark mode CLI theme support (#911)

Add the ability to force dark or light mode colors for CLI output,
overriding the automatic terminal background detection.

Changes:
- Add CLITheme field to TownSettings for persisting preference
- Add GetThemeMode() and HasDarkBackground() to ui package for
  theme detection with GT_THEME env var override
- Add ApplyThemeMode() to explicitly set lipgloss dark background
- Add 'gt theme cli' subcommand to view/set CLI theme preference
- Initialize theme in CLI startup (persistentPreRun)
- Add comprehensive tests for theme mode functionality

Usage:
- gt theme cli              # show current theme
- gt theme cli dark         # force dark mode
- gt theme cli light        # force light mode
- gt theme cli auto         # auto-detect (default)
- GT_THEME=dark gt status   # per-command override

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/ui/styles.go

@@ -22,6 +22,16 @@ func init() {
 	}
 }
 
+// ApplyThemeMode applies the theme mode settings to lipgloss.
+// This should be called after InitTheme() has been called.
+func ApplyThemeMode() {
+	if !ShouldUseColor() {
+		return
+	}
+	// Set lipgloss dark background flag based on theme mode
+	lipgloss.SetHasDarkBackground(HasDarkBackground())
+}
+
 // Ayu theme color palette
 // Dark: https://terminalcolors.com/themes/ayu/dark/
 // Light: https://terminalcolors.com/themes/ayu/light/