fix: handle None genre from Gradio API input

- Default genre to 'fantasy' when Gradio passes None
- Fix generate_procedural_step None check
- Resolve AttributeError: 'NoneType' object has no attribute 'lower'

app.py

@@ -171,7 +171,7 @@ GENRES = {
 
 def generate_procedural_step(genre: str, step: int, health: int, choice: str = "") -> dict:
     """Generate a fallback adventure step without LLM."""
-    genre_data = GENRES.get(genre.lower(), GENRES["fantasy"])
+    genre_data = GENRES.get((genre or "fantasy").lower(), GENRES["fantasy"])
 
     if step == 0:
         return {
@@ -360,8 +360,10 @@ def create_gradio_app() -> gr.Blocks:
         game_over_output = gr.Checkbox(label="Game Over")
         history_output = gr.Textbox(label="History JSON")
 
-        def api_start_game(genre: str):
+        def api_start_game(genre: str = "fantasy"):
             """Start a new interactive text adventure. Exposed as MCP tool."""
+            if not genre:
+                genre = "fantasy"
             genre = genre.lower()
             if genre not in ["fantasy", "scifi", "cyberpunk"]:
                 genre = "fantasy"