feat: 添加移动端下拉菜单和主题色设置

- 新增 MobileMenu 组件，小屏幕使用汉堡菜单替代多个按钮
- 重构 LanguageSettings 为统一下拉框样式
- 新增设置页面，支持主题色切换
- 翻译页添加源语言选择器
- 更新 8 种语言的 i18n 翻译

src/components/theme-provider.tsx

@@ -0,0 +1,76 @@
+"use client";
+
+import { createContext, useContext, useEffect, useState } from "react";
+import {
+  THEME_PRESETS,
+  DEFAULT_THEME,
+  getThemePreset,
+  applyThemeColors,
+  type ThemePreset,
+} from "@/shared/theme-presets";
+
+type ThemeContextType = {
+  currentTheme: string;
+  themePreset: ThemePreset;
+  setTheme: (themeId: string) => void;
+  availableThemes: ThemePreset[];
+};
+
+const ThemeContext = createContext<ThemeContextType | null>(null);
+
+const STORAGE_KEY = "theme-preset";
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [currentTheme, setCurrentTheme] = useState<string>(DEFAULT_THEME);
+  const [mounted, setMounted] = useState(false);
+
+  useEffect(() => {
+    setMounted(true);
+    const savedTheme = localStorage.getItem(STORAGE_KEY);
+    if (savedTheme && getThemePreset(savedTheme)) {
+      setCurrentTheme(savedTheme);
+    }
+  }, []);
+
+  useEffect(() => {
+    if (!mounted) return;
+    const preset = getThemePreset(currentTheme);
+    if (preset) {
+      applyThemeColors(preset);
+      localStorage.setItem(STORAGE_KEY, currentTheme);
+    }
+  }, [currentTheme, mounted]);
+
+  const setTheme = (themeId: string) => {
+    if (getThemePreset(themeId)) {
+      setCurrentTheme(themeId);
+    }
+  };
+
+  const themePreset = getThemePreset(currentTheme) || THEME_PRESETS[0];
+
+  if (!mounted) {
+    return null;
+  }
+
+  return (
+    <ThemeContext.Provider
+      value={{
+        currentTheme,
+        themePreset,
+        setTheme,
+        availableThemes: THEME_PRESETS,
+      }}
+    >
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  const context = useContext(ThemeContext);
+  if (!context) {
+    throw new Error("useTheme must be used within a ThemeProvider");
+  }
+  return context;
+}