feat: add reading position tracking with visual progress indicator

- Install position-indicator library for scroll position tracking
- Create useReadingPosition hook for position management
- Add ReadingProgressIndicator component with animated progress bar
- Integrate reading progress in ContentPanel for text content only
- Add CSS styles for fixed progress indicator with shimmer animation
- Track reading completion at 90% threshold
- Exclude video content from position tracking

src/components/ReadingProgressIndicator.tsx

@@ -0,0 +1,31 @@
+import React from 'react'
+
+interface ReadingProgressIndicatorProps {
+  progress: number // 0 to 100
+  isComplete?: boolean
+  showPercentage?: boolean
+  className?: string
+}
+
+export const ReadingProgressIndicator: React.FC<ReadingProgressIndicatorProps> = ({
+  progress,
+  isComplete = false,
+  showPercentage = true,
+  className = ''
+}) => {
+  return (
+    <div className={`reading-progress-indicator ${className}`}>
+      <div className="reading-progress-bar">
+        <div 
+          className={`reading-progress-fill ${isComplete ? 'complete' : ''}`}
+          style={{ width: `${Math.min(100, Math.max(0, progress))}%` }}
+        />
+      </div>
+      {showPercentage && (
+        <div className="reading-progress-text">
+          {isComplete ? '✓ Complete' : `${progress}%`}
+        </div>
+      )}
+    </div>
+  )
+}