fix: Address code review findings (backend validation, hydration efficiency, plot readability)

backend/src/ea_chatbot/api/routers/agent.py

@@ -91,7 +91,7 @@ async def stream_agent_events(
                         encoded_plots: list[str] = []
                         for fig in plots:
                             buf = io.BytesIO()
-                            fig.savefig(buf, format="png", transparent=True)
+                            fig.savefig(buf, format="png")
                             plot_bytes = buf.getvalue()
                             assistant_plots.append(plot_bytes)
                             encoded_plots.append(base64.b64encode(plot_bytes).decode('utf-8'))

backend/src/ea_chatbot/api/schemas.py

@@ -1,5 +1,5 @@
 from pydantic import BaseModel, EmailStr
-from typing import List, Optional
+from typing import List, Optional, Literal
 from datetime import datetime
 
 # --- Auth Schemas ---
@@ -17,10 +17,10 @@ class UserResponse(BaseModel):
     id: str
     email: str
     display_name: str
-    theme_preference: str
+    theme_preference: Literal["light", "dark"]
 
 class ThemeUpdate(BaseModel):
-    theme: str
+    theme: Literal["light", "dark"]
 
 # --- History Schemas ---
 

backend/src/ea_chatbot/app.py

@@ -420,7 +420,7 @@ def main():
                     st.pyplot(fig)
                     # Convert fig to bytes
                     buf = io.BytesIO()
-                    fig.savefig(buf, format="png", transparent=True)
+                    fig.savefig(buf, format="png")
                     plot_bytes_list.append(buf.getvalue())
             
             if final_state.get("dfs"):