feat: Center-align submit button with input field

UI Improvement:
- Add vertical center alignment for submit button
- Button now aligns with middle of input field
- Better visual balance and user experience

Changes:
1. Add custom CSS for .input-row class
- align-items: center for vertical centering
- height: 100% for button to match input height
- min-height: 42px for minimum button height

2. Update gr.Row with elem_classes="input-row"
- Applies custom CSS styling

3. Add container=False to Textbox
- Removes extra padding for cleaner look

4. Add min_width=80 to Button
- Ensures button has consistent minimum width

Visual Result:
- Before: Button aligned to top of input field
- After: Button vertically centered with input field
- Matches reference image provided by user

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

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

app.py

@@ -465,8 +465,19 @@ def chat_wrapper(message, history):
 
 print(f"✅ App initialized - {HW_ENV['description']}")
 
+# Custom CSS for button alignment
+custom_css = """
+.input-row {
+    align-items: center !important;
+}
+.input-row > div:last-child button {
+    height: 100% !important;
+    min-height: 42px !important;
+}
+"""
+
 # Create Gradio interface
-with gr.Blocks(title="🤖 Multi-Model Chatbot") as demo:
+with gr.Blocks(title="🤖 Multi-Model Chatbot", css=custom_css) as demo:
     # Dynamic header based on hardware environment
     header = f"""
     # 🤖 다중 모델 챗봇 {HW_ENV['description']}
@@ -533,13 +544,14 @@ with gr.Blocks(title="🤖 Multi-Model Chatbot") as demo:
 
     chatbot = gr.Chatbot(height=400, type="messages", show_label=False)
 
-    with gr.Row():
+    with gr.Row(elem_classes="input-row"):
         msg = gr.Textbox(
             placeholder="한글로 메시지를 입력하세요...",
             show_label=False,
             scale=9,
+            container=False,
         )
-        btn = gr.Button("전송", scale=1, variant="primary")
+        btn = gr.Button("전송", scale=1, variant="primary", min_width=80)
 
     clear = gr.Button("🗑️ 대화 초기화", size="sm")