fix: Address second code review findings (test isolation, frontend type safety)

frontend/src/App.tsx

@@ -37,7 +37,7 @@ function App() {
         setUser(userData)
         setIsAuthenticated(true)
         if (userData.theme_preference) {
-          setThemeLocal(userData.theme_preference as "light" | "dark")
+          setThemeLocal(userData.theme_preference)
         }
         // Load history after successful auth
         loadHistory()
@@ -67,7 +67,7 @@ function App() {
       setUser(userData)
       setIsAuthenticated(true)
       if (userData.theme_preference) {
-        setThemeLocal(userData.theme_preference as "light" | "dark")
+        setThemeLocal(userData.theme_preference)
       }
       loadHistory()
     } catch (err: unknown) {

frontend/src/components/theme-provider.tsx

@@ -1,7 +1,7 @@
 import { createContext, useContext, useEffect, useState } from "react"
 import { AuthService } from "@/services/auth"
 
-type Theme = "light" | "dark"
+export type Theme = "light" | "dark"
 
 interface ThemeContextType {
   theme: Theme

frontend/src/services/auth.ts

@@ -1,4 +1,5 @@
 import api from "./api"
+import type { Theme } from "@/components/theme-provider"
 
 export interface AuthResponse {
   access_token: string
@@ -9,7 +10,7 @@ export interface UserResponse {
   id: string
   email: string
   display_name?: string
-  theme_preference: string
+  theme_preference: Theme
 }
 
 export const AuthService = {
@@ -51,7 +52,7 @@ export const AuthService = {
     await api.post("/auth/logout")
   },
 
-  async updateTheme(theme: string): Promise<UserResponse> {
+  async updateTheme(theme: Theme): Promise<UserResponse> {
     const response = await api.patch<UserResponse>("/auth/theme", { theme })
     return response.data
   },