feat(settings): add custom highlight color picker (#2273)

Add ability to customize highlight colors with hex color picker. Users can now set custom colors for all five highlight styles (red, violet, blue, green, yellow) in the settings panel.

Fixes #2271

apps/readest-app/src/services/constants.ts

@@ -78,6 +78,14 @@ export const DEFAULT_SYSTEM_SETTINGS: Partial<SystemSettings> = {
   lastSyncedAtNotes: 0,
 };
 
+export const DEFAULT_CUSTOM_HIGHLIGHT_COLORS: Record<HighlightColor, string> = {
+  red: '#fca5a5', // red-300
+  yellow: '#fde047', // yellow-300
+  green: '#86efac', // green-300
+  blue: '#93c5fd', // blue-300
+  violet: '#c4b5fd', // violet-300
+};
+
 export const DEFAULT_READSETTINGS: ReadSettings = {
   sideBarWidth: '15%',
   isSideBarPinned: true,
@@ -94,6 +102,7 @@ export const DEFAULT_READSETTINGS: ReadSettings = {
     underline: 'green',
     squiggly: 'blue',
   },
+  customHighlightColors: DEFAULT_CUSTOM_HIGHLIGHT_COLORS,
 };
 
 export const DEFAULT_MOBILE_READSETTINGS: Partial<ReadSettings> = {