fix infinite page expansion on mobile

Replace calc(100vh - X) with fixed pixel heights to avoid
iframe expansion loop in HuggingFace Spaces context.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app.py

@@ -736,21 +736,20 @@ CUSTOM_CSS = """
     button.primary { font-weight: 600; }
     .gradio-accordion { margin-bottom: 12px; }
 
-    /* Mobile optimizations */
+    /* Mobile optimizations - use fixed heights to avoid iframe expansion issues */
     @media (max-width: 768px) {
-        /* Constrain the chatbot height on mobile to leave room for input */
+        /* Use fixed height for chatbot on mobile (avoids 100vh iframe issues) */
         #chat-window, .chatbot {
-            height: calc(100vh - 350px) !important;
-            min-height: 200px !important;
-            max-height: calc(100vh - 350px) !important;
+            height: 350px !important;
+            min-height: 250px !important;
+            max-height: 350px !important;
         }
 
-        /* Hide the TTS audio component label on mobile to save space */
+        /* Compact the TTS audio on mobile */
         #tts-audio {
             margin: 4px 0 !important;
         }
 
-        /* Compact the audio player on mobile */
         #tts-audio audio {
             height: 40px !important;
         }
@@ -796,8 +795,8 @@ CUSTOM_CSS = """
     /* Extra small screens (phones in portrait) */
     @media (max-width: 480px) {
         #chat-window, .chatbot {
-            height: calc(100vh - 380px) !important;
-            max-height: calc(100vh - 380px) !important;
+            height: 280px !important;
+            max-height: 280px !important;
         }
 
         #input-row {