feat: Add light/dark mode support with backend persistence

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")