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/App.tsx

@@ -113,11 +113,13 @@ function App() {
 
   return (
     <div className="min-h-screen bg-gray-50">
-      <div className="container mx-auto py-8">
-        <h1 className="text-4xl font-bold text-center mb-8">Text to Audiobook</h1>
+      <div className="container mx-auto py-4 px-4 sm:py-8 sm:px-6 lg:px-8">
+        <h1 className="text-2xl sm:text-3xl md:text-4xl font-bold text-center mb-4 sm:mb-6 md:mb-8">
+          Text to Audiobook
+        </h1>
         
         {error && (
-          <div className="max-w-4xl mx-auto mb-4 p-4 bg-red-100 border border-red-400 text-red-700 rounded">
+          <div className="max-w-4xl mx-auto mb-4 p-3 sm:p-4 bg-red-100 border border-red-400 text-red-700 rounded text-sm sm:text-base">
             {error}
           </div>
         )}
@@ -135,7 +137,7 @@ function App() {
         {phase === 'assignment' && (
           <>
             {isLongText && (
-              <div className="max-w-4xl mx-auto mb-4 p-4 bg-yellow-100 border border-yellow-400 text-yellow-800 rounded">
+              <div className="max-w-4xl mx-auto mb-4 p-3 sm:p-4 bg-yellow-100 border border-yellow-400 text-yellow-800 rounded text-sm sm:text-base">
                 Generation may take several minutes for very long content.
               </div>
             )}
@@ -162,7 +164,7 @@ function App() {
         )}
 
         {isGenerating && isLongText && (
-          <div className="max-w-4xl mx-auto mt-4 p-4 bg-blue-100 border border-blue-400 text-blue-800 rounded text-center">
+          <div className="max-w-4xl mx-auto mt-4 p-3 sm:p-4 bg-blue-100 border border-blue-400 text-blue-800 rounded text-center text-sm sm:text-base">
             Generation may take several minutes for very long content. Please be patient...
           </div>
         )}