feat(theme): add color theme variants for light and dark modes

- Add darkColorTheme: black, midnight (default), charcoal
- Add lightColorTheme: paper-white (default), sepia, ivory
- Extend UserSettings with color theme fields
- Update ThemeSettings UI to show color options
- Add CSS variables for all color theme variants
- Sepia and Ivory have warm, reading-friendly palettes
- Black offers true black for OLED screens
- All color themes sync via Nostr (NIP-78)

src/services/settingsService.ts

@@ -49,6 +49,8 @@ export interface UserSettings {
   autoCollapseSidebarOnMobile?: boolean // Auto-collapse sidebar on mobile (default: true)
   // Theme preference
   theme?: 'dark' | 'light' | 'system' // default: system
+  darkColorTheme?: 'black' | 'midnight' | 'charcoal' // default: midnight
+  lightColorTheme?: 'paper-white' | 'sepia' | 'ivory' // default: paper-white
 }
 
 export async function loadSettings(