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

frontend/src/App.tsx

@@ -12,7 +12,7 @@ import { Button } from "./components/ui/button"
 import { useTheme } from "./components/theme-provider"
 
 function App() {
-  const { setTheme } = useTheme()
+  const { setThemeLocal } = useTheme()
   const [isAuthenticated, setIsAuthenticated] = useState(false)
   const [user, setUser] = useState<UserResponse | null>(null)
   const [authMode, setAuthMode] = useState<"login" | "register">("login")
@@ -37,7 +37,7 @@ function App() {
         setUser(userData)
         setIsAuthenticated(true)
         if (userData.theme_preference) {
-          setTheme(userData.theme_preference as "light" | "dark")
+          setThemeLocal(userData.theme_preference as "light" | "dark")
         }
         // Load history after successful auth
         loadHistory()
@@ -67,7 +67,7 @@ function App() {
       setUser(userData)
       setIsAuthenticated(true)
       if (userData.theme_preference) {
-        setTheme(userData.theme_preference as "light" | "dark")
+        setThemeLocal(userData.theme_preference as "light" | "dark")
       }
       loadHistory()
     } catch (err: unknown) {

frontend/src/components/theme-provider.tsx

@@ -6,6 +6,7 @@ type Theme = "light" | "dark"
 interface ThemeContextType {
   theme: Theme
   setTheme: (theme: Theme) => void
+  setThemeLocal: (theme: Theme) => void
   toggleTheme: () => void
 }
 
@@ -35,12 +36,16 @@ export function ThemeProvider({
     }
   }
 
+  const setThemeLocal = (newTheme: Theme) => {
+    setThemeState(newTheme)
+  }
+
   const toggleTheme = () => {
     setTheme(theme === "light" ? "dark" : "light")
   }
 
   return (
-    <ThemeContext.Provider value={{ theme, setTheme, toggleTheme }}>
+    <ThemeContext.Provider value={{ theme, setTheme, setThemeLocal, toggleTheme }}>
       {children}
     </ThemeContext.Provider>
   )