feat: Add light/dark mode support with backend persistence

frontend/src/App.tsx

@@ -9,8 +9,10 @@ import { ChatService, type MessageResponse } from "./services/chat"
 import { type Conversation } from "./components/layout/HistorySidebar"
 import { registerUnauthorizedCallback } from "./services/api"
 import { Button } from "./components/ui/button"
+import { useTheme } from "./components/theme-provider"
 
 function App() {
+  const { setTheme } = useTheme()
   const [isAuthenticated, setIsAuthenticated] = useState(false)
   const [user, setUser] = useState<UserResponse | null>(null)
   const [authMode, setAuthMode] = useState<"login" | "register">("login")
@@ -34,6 +36,9 @@ function App() {
         const userData = await AuthService.getMe()
         setUser(userData)
         setIsAuthenticated(true)
+        if (userData.theme_preference) {
+          setTheme(userData.theme_preference as "light" | "dark")
+        }
         // Load history after successful auth
         loadHistory()
       } catch (err: unknown) {
@@ -61,6 +66,9 @@ function App() {
       const userData = await AuthService.getMe()
       setUser(userData)
       setIsAuthenticated(true)
+      if (userData.theme_preference) {
+        setTheme(userData.theme_preference as "light" | "dark")
+      }
       loadHistory()
     } catch (err: unknown) {
       console.error("Failed to fetch user profile after login:", err)