Complete model selector sidebar move for better UX

Move model selector from header to top of sidebar:
- No more scrolling to top to change models
- Always visible while browsing questions
- Perfect for using retry button with different models
- Clean main header with just title
- Better workflow: pick model → choose question → see results → retry

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

Co-Authored-By: Claude <[email protected]>

app.py

@@ -572,17 +572,8 @@ if "deepseek-R1" in available_models:
 elif "gpt-oss-120b" in available_models:
     default_index = available_models.index("gpt-oss-120b")
 
-# Simple header - just title and model selector
-col1, col2 = st.columns([3, 1])
-with col1:
-    st.title("VayuChat")
-with col2:
-    model_name = st.selectbox(
-        "Model:",
-        available_models,
-        index=default_index,
-        help="Choose your AI model"
-    )
+# Simple header - just title
+st.title("VayuChat")
 
 
 # Load data with caching for better performance
@@ -602,8 +593,18 @@ image_path = "IITGN_Logo.png"
 
 # Clean sidebar  
 with st.sidebar:
-    # Quick Queries Section - moved to top
-    st.markdown("### Quick Queries")
+    # Model selector at top of sidebar for easy access
+    model_name = st.selectbox(
+        "🤖 AI Model:",
+        available_models,
+        index=default_index,
+        help="Choose your AI model - easily accessible without scrolling!"
+    )
+    
+    st.markdown("---")
+    
+    # Quick Queries Section
+    st.markdown("### 💭 Quick Queries")
     
     # Load quick prompts with caching
     @st.cache_data