refactor: switch to brighter yellow-300 for highlight defaults and add semantic color aliases

src/components/Settings/ReadingDisplaySettings.tsx

@@ -73,7 +73,7 @@ const ReadingDisplaySettings: React.FC<ReadingDisplaySettingsProps> = ({ setting
         <label className="setting-label">My Highlights</label>
         <div className="setting-control">
           <ColorPicker
-            selectedColor={settings.highlightColorMine || '#facc15'}
+            selectedColor={settings.highlightColorMine || '#fde047'}
             onColorChange={(color) => onUpdate({ highlightColorMine: color })}
           />
         </div>

src/components/ThreePaneLayout.tsx

@@ -252,7 +252,7 @@ const ThreePaneLayout: React.FC<ThreePaneLayoutProps> = (props) => {
             right: 'calc(1rem + env(safe-area-inset-right))',
             width: '40px',
             height: '40px',
-            backgroundColor: `${props.settings.highlightColorMine || '#facc15'}B3`,
+            backgroundColor: `${props.settings.highlightColorMine || '#fde047'}B3`,
             color: '#000'
           }}
           onClick={props.onToggleHighlightsPanel}