Update citation to Zenodo DOI and change font to Helvetica

- Updated About section citation with DOI: 10.5281/zenodo.15532316
- Added custom CSS to use Helvetica font throughout the app

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app.py

@@ -774,7 +774,13 @@ result.to_csv("classified_results.csv", index=False)
     return gr.update(value=code, visible=True)
 
 
-with gr.Blocks(title="CatLLM - Survey Response Classifier", theme=gr.themes.Soft()) as demo:
+custom_css = """
+* {
+    font-family: Helvetica, Arial, sans-serif !important;
+}
+"""
+
+with gr.Blocks(title="CatLLM - Survey Response Classifier", theme=gr.themes.Soft(), css=custom_css) as demo:
     gr.Image("logo.png", show_label=False, show_download_button=False, height=100, container=False)
     gr.Markdown("# CatLLM - Survey Response Classifier")
     gr.Markdown("Classify survey responses into custom categories using LLMs.")
@@ -805,8 +811,7 @@ This app is currently in **beta** and **free to use** while CatLLM is under revi
 ### Citation
 If you use CatLLM in your research, please cite:
 ```
-Soria, C. (2025). CatLLM: A Python package for LLM-based text classification.
-https://github.com/chrissoria/cat-llm
+Soria, C. (2025). CatLLM: A Python package for LLM-based text classification. DOI: 10.5281/zenodo.15532316
 ```
 """)