Adds Settings > Appearance > Color Theme with visual swatch selector.

Themes:
- Default  (Material You purple/blue)
- GitHub   (GitHub blue #0969DA / #58A6FF)
- Catppuccin (Latte light / Mocha dark — mauve accent)
- Dracula  (purple #BD93F9 / #6272A4)
- Nord     (arctic frost #88C0D0 / #5E81AC)
- Rosé Pine (rose #EBBCBA / #B4637A)
- Ayu      (orange #FFB454 / #FF8F40)

Changes:
- ColorTheme enum in PreferenceEnums
- colorTheme pref in PreferenceManager (key: color_theme)
- ThemeColors.kt: light + dark ColorScheme per theme + toColorScheme()
- GloomTheme: colorTheme param threaded through expect/actual
- App.kt: passes prefs.colorTheme to GloomTheme
- ColorThemePicker: FlowRow of clickable swatches with check icon
- AppearanceSettingsScreen: shows picker below dark/light toggle
  (hidden when Monet/dynamic color is active)