feat: 新增remote-only主题模式，优化首屏包体积

1. 新增theme-override.js实现主题远端拉取逻辑
2. 新增empty-style-stub.js作为主题包占位符，减少主包~75KB
3. 在vue.config.js中添加构建期主题stub替换逻辑
4. 重构config-external.js的主题配置，新增remote-only模式支持
5. 重构store的applyThemeWithRemote方法，适配远端主题拉取
6. 在bootstrap中新增主题初始化和异步拉取逻辑

main.js

@@ -17,6 +17,7 @@ import {
 import configExternal from './common/js/config-external.js'
 import PrivacyManager from './common/js/privacy.js'
 import splashState from './common/js/splash-state.js'
+import ThemeOverride from './common/js/theme-override.js'
 
 Vue.prototype.$store = store //挂在vue
 
@@ -105,7 +106,15 @@ async function bootstrap() {
 	if (Lang.isRemoteOnly && Lang.isRemoteOnly()) {
 		splashState.show();
 		try {
+			// 1) i18n: 拉远端语言包 (langConfig.source === 'remote-only')
 			await Lang.boot();
+			// 2) theme: 阻塞式预热 brand 桶覆写 (themeConfig.overrideRemote.source)
+			await ThemeOverride.boot();
+			// 3) theme(remote-only): 异步拉全量 8 个 preset 替换 stub
+			//    失败静默, 兜底用 webpack stub(只含 default),首屏不白屏
+			ThemeOverride.loadAllPresets().catch((e) => {
+				console.warn('[theme-presets] load fail', e);
+			});
 		} catch (e) {
 			console.warn('[i18n] remote-only boot fail', e);
 		} finally {