feat: Add responsive styling and mobile support

- Applied Tailwind CSS responsive classes across all components
- Implemented mobile-first design approach with breakpoints (sm, md, lg)
- Added touch-friendly interactions with min-h-[44px] touch targets
- Enhanced App.tsx with responsive padding, headings, and alerts
- Updated TextInputSection with responsive textarea and full-width mobile button
- Improved VoiceAssignmentSection with flexible layouts and mobile-optimized controls
- Enhanced AudioPlayerSection with responsive sizing and touch support
- Added custom CSS for range slider with larger thumb for mobile
- Improved select dropdowns with visible arrows and proper padding
- Added touch-manipulation class to prevent double-tap zoom
- Added active states for better mobile feedback
- Updated index.html with proper title and meta description
- Created RESPONSIVE_IMPROVEMENTS.md documentation

Requirements: 12.1, 12.2, 12.3, 12.4

frontend/src/components/AudioPlayerSection.tsx

@@ -8,15 +8,15 @@ export default function AudioPlayerSection({
   onDownload
 }: AudioPlayerSectionProps) {
   return (
-    <div className="w-full max-w-4xl mx-auto p-6">
-      <h2 className="text-2xl font-bold mb-6">Your Audiobook is Ready!</h2>
+    <div className="w-full max-w-4xl mx-auto p-4 sm:p-6">
+      <h2 className="text-xl sm:text-2xl font-bold mb-4 sm:mb-6">Your Audiobook is Ready!</h2>
       
-      <div className="bg-white rounded-lg shadow-md p-6 mb-6">
-        <h3 className="text-lg font-semibold mb-4">Listen to your audiobook</h3>
+      <div className="bg-white rounded-lg shadow-md p-4 sm:p-6 mb-4 sm:mb-6">
+        <h3 className="text-base sm:text-lg font-semibold mb-3 sm:mb-4">Listen to your audiobook</h3>
         <audio
           controls
           src={audioUrl}
-          className="w-full"
+          className="w-full min-h-[44px]"
           preload="metadata"
         >
           Your browser does not support the audio element.
@@ -26,7 +26,7 @@ export default function AudioPlayerSection({
       <div className="flex justify-center">
         <button
           onClick={onDownload}
-          className="px-6 py-3 bg-green-600 hover:bg-green-700 text-white font-semibold rounded-lg transition-colors cursor-pointer"
+          className="w-full sm:w-auto px-6 py-3 min-h-[44px] bg-green-600 hover:bg-green-700 active:bg-green-800 text-white font-semibold rounded-lg transition-colors cursor-pointer touch-manipulation"
         >
           Download audiobook
         </button>