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/TextInputSection.tsx

@@ -36,21 +36,21 @@ export default function TextInputSection({
   const isButtonDisabled = disabled || isLoading || text.trim() === '';
 
   return (
-    <div className="w-full max-w-4xl mx-auto p-6">
-      <h2 className="text-2xl font-bold mb-4">Enter Your Text</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-3 sm:mb-4">Enter Your Text</h2>
       
       <div className="mb-4">
         <textarea
           value={text}
           onChange={handleTextChange}
           placeholder="Enter your text here. Use patterns like 'Name:' or '[Name]' to mark different speakers..."
-          className="w-full h-96 p-4 border-2 border-gray-300 rounded-lg focus:border-blue-500 focus:outline-none resize-y text-base"
+          className="w-full h-64 sm:h-80 md:h-96 p-3 sm:p-4 border-2 border-gray-300 rounded-lg focus:border-blue-500 focus:outline-none resize-y text-sm sm:text-base touch-manipulation"
           disabled={disabled || isLoading}
         />
       </div>
 
       {showValidation && text.trim() === '' && (
-        <div className="mb-4 p-3 bg-red-100 border border-red-400 text-red-700 rounded">
+        <div className="mb-4 p-3 bg-red-100 border border-red-400 text-red-700 rounded text-sm sm:text-base">
           Please enter some text before proceeding.
         </div>
       )}
@@ -58,10 +58,10 @@ export default function TextInputSection({
       <button
         onClick={handleNext}
         disabled={isButtonDisabled}
-        className={`px-6 py-3 rounded-lg font-semibold text-white transition-colors ${
+        className={`w-full sm:w-auto px-6 py-3 min-h-[44px] rounded-lg font-semibold text-white transition-colors touch-manipulation ${
           isButtonDisabled
             ? 'bg-gray-400 cursor-not-allowed'
-            : 'bg-blue-600 hover:bg-blue-700 cursor-pointer'
+            : 'bg-blue-600 hover:bg-blue-700 active:bg-blue-800 cursor-pointer'
         }`}
       >
         {isLoading ? 'Processing...' : 'Next: Assign voices'}