refactor(theme): change default light theme to sepia

- Update default from paper-white to sepia for warmer reading
- Midnight remains default for dark mode
- Sepia provides warm, eye-friendly tones for light mode

src/utils/theme.ts

@@ -19,7 +19,7 @@ export function getSystemTheme(): 'dark' | 'light' {
 export function applyTheme(
   theme: Theme,
   darkColorTheme: DarkColorTheme = 'midnight',
-  lightColorTheme: LightColorTheme = 'paper-white'
+  lightColorTheme: LightColorTheme = 'sepia'
 ): void {
   const root = document.documentElement