feat: add pulsing animation when scrolling to highlight

- Replace brightness change with subtle pulse animation
- Pulse twice over 1.5 seconds with scale and glow effects
- Scale slightly (1.02x) and increase shadow glow
- More elegant visual feedback than color change
- Easier to spot without being jarring

The highlight now pulses twice when clicked from the
sidebar, making it easy to see where you've jumped to.

src/components/ContentPanel.tsx

@@ -44,12 +44,12 @@ const ContentPanel: React.FC<ContentPanelProps> = ({
       console.log('📍 Scrolling to highlight in article:', selectedHighlightId.slice(0, 8))
       markElement.scrollIntoView({ behavior: 'smooth', block: 'center' })
       
-      // Temporarily enhance the highlight to show it's selected
-      const originalBackground = (markElement as HTMLElement).style.background
-      ;(markElement as HTMLElement).style.background = 'rgba(255, 255, 0, 0.7)'
+      // Add pulsing animation to show it's selected
+      const htmlElement = markElement as HTMLElement
+      htmlElement.classList.add('highlight-pulse')
       
       setTimeout(() => {
-        (markElement as HTMLElement).style.background = originalBackground
+        htmlElement.classList.remove('highlight-pulse')
       }, 1500)
     } else {
       console.log('⚠️ Could not find mark element for highlight:', selectedHighlightId.slice(0, 8))

src/index.css

@@ -1374,6 +1374,29 @@ body {
   box-shadow: 0 0 12px rgba(255, 255, 0, 0.3);
 }
 
+.content-highlight.highlight-pulse {
+  animation: highlight-pulse-animation 1.5s ease-in-out;
+}
+
+@keyframes highlight-pulse-animation {
+  0%, 100% {
+    box-shadow: 0 0 8px rgba(255, 255, 0, 0.2);
+    transform: scale(1);
+  }
+  25% {
+    box-shadow: 0 0 20px rgba(255, 255, 0, 0.6);
+    transform: scale(1.02);
+  }
+  50% {
+    box-shadow: 0 0 8px rgba(255, 255, 0, 0.2);
+    transform: scale(1);
+  }
+  75% {
+    box-shadow: 0 0 20px rgba(255, 255, 0, 0.6);
+    transform: scale(1.02);
+  }
+}
+
 .reader-html .content-highlight,
 .reader-markdown .content-highlight {
   color: inherit;