feat: 暗黑模式 + 节流模式

- settingsStore: 移除 coverQuality，新增 darkMode / trafficSaving 布尔字段（AsyncStorage 持久化）
- utils/theme.ts: 新建 useTheme() hook，返回浅色/深色颜色表
- settings.tsx: 新增外观（浅色/深色）和流量（标准/节流）选项，移除封面图清晰度
- BigVideoCard: 节流模式跳过预加载、禁止自动播放；封面低画质；支持暗黑配色
- useVideoDetail: 节流模式下默认 qn=16（360p）
- VideoCard / LiveCard: 移除 coverQuality 依赖，改用 trafficSaving；支持暗黑配色
- index.tsx: 导航栏/搜索栏/列表背景跟随主题
- search.tsx: 页面背景/输入框/文字跟随主题
- downloads.tsx: 顶栏/背景/文字跟随主题
- _layout.tsx: StatusBar 随暗黑模式切换 light/dark

hooks/useVideoDetail.ts

@@ -1,6 +1,7 @@
 import { useState, useEffect, useRef } from 'react';
 import { getVideoDetail, getPlayUrl } from '../services/bilibili';
 import { useAuthStore } from '../store/authStore';
+import { useSettingsStore } from '../store/settingsStore';
 import type { VideoItem, PlayUrlResponse } from '../services/types';
 
 export function useVideoDetail(bvid: string) {
@@ -12,6 +13,8 @@ export function useVideoDetail(bvid: string) {
   const [error, setError] = useState<string | null>(null);
   const cidRef = useRef<number>(0);
   const isLoggedIn = useAuthStore(s => s.isLoggedIn);
+  const trafficSaving = useSettingsStore(s => s.trafficSaving);
+  const defaultQn = trafficSaving ? 16 : 126;
 
   async function fetchPlayData(cid: number, qn: number, updateList = false) {
     const data = await getPlayUrl(bvid, cid, qn);
@@ -39,7 +42,7 @@ export function useVideoDetail(bvid: string) {
         setVideo(detail);
         const cid = detail.pages?.[0]?.cid ?? detail.cid as number;
         cidRef.current = cid;
-        await fetchPlayData(cid, 126, true);
+        await fetchPlayData(cid, defaultQn, true);
       } catch (e: any) {
         setError(e.message ?? 'Load failed');
       } finally {
@@ -52,7 +55,7 @@ export function useVideoDetail(bvid: string) {
   // 登录状态变化时重新拉取清晰度列表（登录后可能获得更高画质）
   useEffect(() => {
     if (cidRef.current) {
-      fetchPlayData(cidRef.current, 126, true).catch((e) => {
+      fetchPlayData(cidRef.current, defaultQn, true).catch((e) => {
         console.warn('Failed to refresh quality list after login change:', e);
       });
     }