Fix gradio 6.19 launch() compatibility

Deployed Space installed gradio 6.19.0 (within the >=6.0,<7 range), where
show_api was fully removed from Blocks.launch() (only deprecated as of
the 6.13.0 I tested locally) -- caused a hard startup TypeError. Also
proactively move theme from the Blocks constructor to launch(), since
it hit the same deprecation-then-removal pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app.py

@@ -167,7 +167,7 @@ def make_analyze_handler(slot_index: int):
 
 
 def build_ui() -> gr.Blocks:
-    with gr.Blocks(title="GuestPostSuggester", theme=gr.themes.Soft()) as demo:
+    with gr.Blocks(title="GuestPostSuggester") as demo:
         gr.Markdown(INTRO)
         with gr.Row():
             topic_box = gr.Textbox(label="Topic", placeholder="e.g. sustainable travel", scale=4)
@@ -224,4 +224,8 @@ def build_ui() -> gr.Blocks:
 
 
 if __name__ == "__main__":
-    build_ui().queue().launch(server_name="0.0.0.0", server_port=7860, show_api=False)
+    # show_api was deprecated in gradio 6.13 and removed entirely by 6.19 — don't pass it.
+    # theme moved from the Blocks constructor to launch() as of gradio 6.0.
+    build_ui().queue().launch(
+        server_name="0.0.0.0", server_port=7860, theme=gr.themes.Soft(),
+    )