Generalize theme system from Catppuccin-only (4 flavors) to 11 themes
across 2 groups. New editor themes: VSCode Dark+, Atom One Dark,
Monokai, Dracula, Nord, Solarized Dark, GitHub Dark.
All themes map to the same 26 --ctp-* CSS custom properties, so every
component works unchanged. ThemeId replaces CatppuccinFlavor as primary
type. Theme store uses getCurrentTheme()/setTheme() with deprecated
wrappers. SettingsTab uses optgroup-based theme selector.
ff2d354219