Add voice preview and age category modifiers

Features:
- Voice preview button to test voices before generation
- Age category selection (young, teen, adult, old)
- Age categories adjust voice speed automatically:
  - Young: 1.15x (faster, energetic)
  - Teen: 1.08x (slightly faster)
  - Adult: 1.0x (normal)
  - Old: 0.85x (slower, measured)
- Preview caching to avoid regenerating same voice
- Backend /api/tts/preview endpoint for voice samples
- Frontend previewVoice API function

This allows users to hear how each voice sounds and adjust
character ages to make them sound younger or older.

backend/main.py

@@ -62,6 +62,13 @@ class TTSRequest(BaseModel):
     roles: List[RoleVoiceMapping] = Field(..., description="Role to voice mappings")
     speed: Optional[float] = Field(1.0, description="Playback speed (0.5-1.5)")
 
+class VoicePreviewRequest(BaseModel):
+    """Request model for voice preview endpoint."""
+    voice: str = Field(..., description="Kokoro voice ID to preview")
+    lang: str = Field(..., description="Language code (a, b, f, j)")
+    speed: Optional[float] = Field(1.0, description="Playback speed (0.5-1.5)")
+    text: Optional[str] = Field(None, description="Custom text to preview (uses default if not provided)")
+
 class ErrorResponse(BaseModel):
     """Standard error response model."""
     error: str = Field(..., description="Error type")
@@ -234,6 +241,90 @@ async def generate_script(request: ScriptRequest):
             }
         )
 
+@app.post("/api/tts/preview")
+async def preview_voice(request: VoicePreviewRequest):
+    """
+    Generates a short audio preview of a voice.
+    
+    Args:
+        request: VoicePreviewRequest containing voice ID, language, speed, and optional text
+        
+    Returns:
+        StreamingResponse with audio/wav content
+        
+    Raises:
+        400: If validation fails
+        500: If audio generation fails
+    """
+    logger.info(f"Voice preview requested: voice={request.voice}, lang={request.lang}, speed={request.speed}")
+    
+    # Validate language code
+    supported_langs = ['a', 'b', 'e', 'f', 'h', 'i', 'j', 'p', 'z']
+    if request.lang not in supported_langs:
+        logger.error(f"Invalid language code: {request.lang}")
+        return JSONResponse(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            content={
+                "error": "ValidationError",
+                "message": f"Invalid language code: {request.lang}",
+                "details": {"field": "lang", "supported": supported_langs}
+            }
+        )
+    
+    # Validate speed range
+    if request.speed < 0.5 or request.speed > 1.5:
+        logger.error(f"Invalid speed value: {request.speed}")
+        return JSONResponse(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            content={
+                "error": "ValidationError",
+                "message": "Speed must be between 0.5 and 1.5",
+                "details": {"field": "speed", "value": request.speed, "range": [0.5, 1.5]}
+            }
+        )
+    
+    # Use provided text or default preview text
+    preview_text = request.text or "Hello! This is a preview of this voice. How does it sound?"
+    
+    try:
+        # Get pipeline for language
+        pipeline = tts_service.get_pipeline(request.lang)
+        
+        # Generate preview audio
+        audio = tts_service.synthesize_segment(
+            pipeline=pipeline,
+            text=preview_text,
+            voice=request.voice,
+            speed=request.speed
+        )
+        
+        # Convert to WAV bytes
+        import io
+        import soundfile as sf
+        buffer = io.BytesIO()
+        sf.write(buffer, audio, tts_service.SAMPLE_RATE, format='WAV')
+        buffer.seek(0)
+        audio_bytes = buffer.read()
+        
+        logger.info(f"Generated voice preview: {len(audio_bytes)} bytes")
+        
+        return StreamingResponse(
+            iter([audio_bytes]),
+            media_type="audio/wav",
+            headers={"Content-Disposition": "inline; filename=preview.wav"}
+        )
+        
+    except Exception as e:
+        logger.error(f"Error during voice preview: {str(e)}", exc_info=True)
+        return JSONResponse(
+            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
+            content={
+                "error": "PreviewGenerationError",
+                "message": "Failed to generate voice preview",
+                "details": {"error": str(e)}
+            }
+        )
+
 @app.post("/api/tts")
 async def generate_audiobook(request: TTSRequest):
     """