Replace accent presets with the sunset-to-sea family

Swap Toffee/Slate/Evergreen/Lavender for rose/coral/amber/teal — four ramps
sharing one lightness/saturation curve (src/renderer/src/theme.ts). Default
accent is now teal. Updates ACCENT_COLORS (src/shared/ipc.ts), the settings
default + stale-value coercion (src/main/settings.ts), the dev/store fallbacks
(main.tsx, store/settings.ts), and the native window background to match the
new pageBackground.light (src/main/index.ts).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/index.ts

@@ -63,7 +63,7 @@ let mainWindow: BrowserWindow | null = null
 // src/renderer/src/theme.ts. Set as the window's NATIVE background so the
 // one-frame compositor repaint (when a tooltip/dropdown overlay first paints on
 // Windows) shows the app's current color instead of a mismatched white flash.
-const THEME_BACKGROUND = { light: '#f6f1ef', dark: '#161618' } as const
+const THEME_BACKGROUND = { light: '#f7f7f8', dark: '#161618' } as const
 
 // 'system' isn't a real background — resolve it against the OS's current
 // preference (nativeTheme.themeSource defaults to 'system', so this tracks it