Fix font: add sans-serif fallback chain for Inter

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app.py

@@ -289,7 +289,8 @@ theme = gr.themes.Base(
     primary_hue=gr.themes.colors.amber,
     secondary_hue=gr.themes.colors.stone,
     neutral_hue=gr.themes.colors.stone,
-    font=gr.themes.GoogleFont("Inter"),
+    font=[gr.themes.GoogleFont("Inter"), "system-ui", "sans-serif"],
+    font_mono=[gr.themes.GoogleFont("JetBrains Mono"), "monospace"],
 ).set(
     body_background_fill="#f5f3ee",
     block_background_fill="#ffffff",