refactor: 移除主题切换功能，统一使用明亮风格

- 从 Header 移除 ThemeToggle 组件
- 删除 theme-toggle.tsx 文件
- 简化 theme-context.tsx，只保留 light 主题~

src/contexts/theme-context.tsx

@@ -1,65 +1,17 @@
 'use client';
 
-import { createContext, useContext, useEffect, useState, type ReactNode } from 'react';
-
-type Theme = 'light' | 'dark' | 'system';
+import { createContext, useContext, type ReactNode } from 'react';
 
 interface ThemeContextType {
-  theme: Theme;
-  setTheme: (theme: Theme) => void;
-  resolvedTheme: 'light' | 'dark';
+  theme: 'light';
+  resolvedTheme: 'light';
 }
 
 const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
 
-const THEME_STORAGE_KEY = 'ruixin-theme';
-
 export function ThemeProvider({ children }: { children: ReactNode }) {
-  const [theme, setThemeState] = useState<Theme>('system');
-  const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('light');
-
-  useEffect(() => {
-    const stored = localStorage.getItem(THEME_STORAGE_KEY) as Theme | null;
-    if (stored && ['light', 'dark', 'system'].includes(stored)) {
-      setThemeState(stored);
-    }
-  }, []);
-
-  useEffect(() => {
-    const root = document.documentElement;
-    const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
-
-    const updateTheme = () => {
-      let resolved: 'light' | 'dark';
-
-      if (theme === 'system') {
-        resolved = mediaQuery.matches ? 'dark' : 'light';
-      } else {
-        resolved = theme;
-      }
-
-      setResolvedTheme(resolved);
-
-      if (resolved === 'dark') {
-        root.classList.add('dark');
-      } else {
-        root.classList.remove('dark');
-      }
-    };
-
-    updateTheme();
-
-    mediaQuery.addEventListener('change', updateTheme);
-    return () => mediaQuery.removeEventListener('change', updateTheme);
-  }, [theme]);
-
-  const setTheme = (newTheme: Theme) => {
-    setThemeState(newTheme);
-    localStorage.setItem(THEME_STORAGE_KEY, newTheme);
-  };
-
   return (
-    <ThemeContext.Provider value={{ theme, setTheme, resolvedTheme }}>
+    <ThemeContext.Provider value={{ theme: 'light', resolvedTheme: 'light' }}>
       {children}
     </ThemeContext.Provider>
   );