feat: Add light/dark mode support with backend persistence

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")
+                            fig.savefig(buf, format="png", transparent=True)
                             plot_bytes = buf.getvalue()
                             assistant_plots.append(plot_bytes)
                             encoded_plots.append(base64.b64encode(plot_bytes).decode('utf-8'))

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

@@ -4,7 +4,7 @@ from fastapi.security import OAuth2PasswordRequestForm
 from ea_chatbot.api.utils import create_access_token, settings
 from ea_chatbot.api.dependencies import history_manager, oidc_client, get_current_user
 from ea_chatbot.history.models import User as UserDB
-from ea_chatbot.api.schemas import Token, UserCreate, UserResponse
+from ea_chatbot.api.schemas import Token, UserCreate, UserResponse, ThemeUpdate
 from ea_chatbot.auth import OIDCSession
 import logging
 
@@ -45,7 +45,8 @@ async def register(user_in: UserCreate, response: Response):
     return {
         "id": str(user.id),
         "email": user.username,
-        "display_name": user.display_name
+        "display_name": user.display_name,
+        "theme_preference": user.theme_preference
     }
 
 @router.post("/login", response_model=Token)
@@ -155,5 +156,25 @@ async def get_me(current_user: UserDB = Depends(get_current_user)):
     return {
         "id": str(current_user.id),
         "email": current_user.username,
-        "display_name": current_user.display_name
+        "display_name": current_user.display_name,
+        "theme_preference": current_user.theme_preference
+    }
+
+@router.patch("/theme", response_model=UserResponse)
+async def update_theme(
+    theme_in: ThemeUpdate, 
+    current_user: UserDB = Depends(get_current_user)
+):
+    """Update the current user's theme preference."""
+    user = history_manager.update_user_theme(current_user.id, theme_in.theme)
+    if not user:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="User not found"
+        )
+    return {
+        "id": str(user.id),
+        "email": user.username,
+        "display_name": user.display_name,
+        "theme_preference": user.theme_preference
     }

backend/src/ea_chatbot/api/schemas.py

@@ -17,6 +17,10 @@ class UserResponse(BaseModel):
     id: str
     email: str
     display_name: str
+    theme_preference: str
+
+class ThemeUpdate(BaseModel):
+    theme: str
 
 # --- 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")
+                    fig.savefig(buf, format="png", transparent=True)
                     plot_bytes_list.append(buf.getvalue())
             
             if final_state.get("dfs"):

backend/src/ea_chatbot/history/manager.py

@@ -70,6 +70,16 @@ class HistoryManager:
         except VerifyMismatchError:
             return None
 
+    def update_user_theme(self, user_id: str, theme: str) -> Optional[User]:
+        """Update the user's theme preference."""
+        with self.get_session() as session:
+            user = session.get(User, user_id)
+            if user:
+                user.theme_preference = theme
+                session.commit()
+                session.refresh(user)
+            return user
+
     def sync_user_from_oidc(self, email: str, display_name: Optional[str] = None) -> User:
         """
         Synchronize a user from an OIDC provider. 

backend/src/ea_chatbot/history/models.py

@@ -14,6 +14,7 @@ class User(Base):
     username: Mapped[str] = mapped_column(String, unique=True, index=True)
     password_hash: Mapped[Optional[str]] = mapped_column(String, nullable=True)
     display_name: Mapped[Optional[str]] = mapped_column(String, nullable=True)
+    theme_preference: Mapped[str] = mapped_column(String, default="light")
     
     conversations: Mapped[List["Conversation"]] = relationship(back_populates="user", cascade="all, delete-orphan")