feat: 全屏「正在播放」页与跨平台兜底播放

- NowPlaying：参考 Apple Music tvOS —— 模糊封面铺底、大号逐行歌词（按距离渐隐 / 模糊、逐行跟随）、接下来列表、可拖动进度条，静置后隐藏控件；Esc 收起、←/→ 快退 / 快进
- 播放兜底链：曲库文件 → 原平台 → 另一个平台的同名歌曲，出错时依次切换并保留进度，合并为一条提示
- 新增 /api/:source/stream-alt：按歌名、歌手、时长在另一个平台匹配同一首歌并代理播放

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/player.ts

@@ -23,8 +23,15 @@ export interface PlayItem {
   local?: boolean;
   /** 毫秒，加载前先显示 */
   duration?: number;
+  /** src 播放失败时依次尝试：曲库文件 / 原平台 / 另一个平台的同名歌曲 */
+  alt?: Array<{ src: string; via: Via }>;
 }
 
+/** 实际出声的音源 */
+export type Via = 'local' | Source;
+export const VIA_LABEL: Record<Via, string> = { local: '曲库文件', netease: '网易云', qq: 'QQ 音乐' };
+const OTHER: Record<Source, Source> = { netease: 'qq', qq: 'netease' };
+
 export type PlayMode = 'list' | 'loop' | 'one' | 'shuffle';
 export const MODE_LABEL: Record<PlayMode, string> = { list: '顺序播放', loop: '列表循环', one: '单曲循环', shuffle: '随机播放' };
 const MODES: PlayMode[] = ['loop', 'one', 'shuffle', 'list'];
@@ -48,6 +55,10 @@ export const player = reactive({
   panel: null as null | 'queue' | 'lyric',
   /** 当前队列的来源，如 playlist:3，用于在来源页面上显示播放状态 */
   context: '',
+  /** 当前实际播放的音源（兜底后可能与曲目来源不同） */
+  via: 'local' as Via,
+  /** 全屏「正在播放」 */
+  full: false,
 });
 
 /** 播放队列可能有上千首：用 shallowRef，整体替换触发更新 */
@@ -60,6 +71,10 @@ let attached = false;
 /** 恢复上次进度：元数据加载后跳转 */
 let resumeAt = 0;
 let errorStreak = 0;
+/** 当前曲目已用掉的兜底音源数 */
+let attempt = 0;
+/** 本首已失败的音源，换源后真正出声时合并成一条提示 */
+let failedVias: Via[] = [];
 let prefetched = '';
 /** 播放顺序 → 队列下标；随机模式下是洗牌后的顺序 */
 let order: number[] = [];
@@ -128,8 +143,13 @@ function loadLyric(item: PlayItem) {
 
 // ---------- 曲目转换 ----------
 
+/** 曲库优先（默认）：曲库文件 → 原平台 → 另一个平台；关闭时原平台排在曲库文件前 */
 export function fromTrack(t: TrackSummary): PlayItem {
-  const local = settings.value?.preferLocal !== false ? t.localFile : undefined;
+  const online = { src: api.streamUrl(t.source, t.id), via: t.source as Via };
+  const file = t.localFile ? { src: api.libraryFileUrl(t.localFile), via: 'local' as Via } : null;
+  const chain = (settings.value?.preferLocal !== false ? [file, online] : [online, file]).filter((x) => !!x);
+  chain.push({ src: api.altStreamUrl(t.source, { name: t.name, artists: t.artists, duration: t.duration }), via: OTHER[t.source] });
+  const [first, ...alt] = chain;
   return {
     key: `${t.source}:${t.id}`,
     title: t.name,
@@ -140,8 +160,9 @@ export function fromTrack(t: TrackSummary): PlayItem {
     id: t.id,
     albumId: t.albumId,
     duration: t.duration,
-    src: local ? api.libraryFileUrl(local) : api.streamUrl(t.source, t.id),
-    local: !!local,
+    src: first!.src,
+    local: first!.via === 'local',
+    alt,
   };
 }
 
@@ -206,6 +227,7 @@ function restore() {
       queue.value = s.queue.filter((i: PlayItem) => i && typeof i.key === 'string' && typeof i.src === 'string');
       player.index = Math.min(Math.max(0, Number(s.index) || 0), queue.value.length - 1);
       player.current = queue.value[player.index] ?? null;
+      if (player.current) player.via = primaryVia(player.current);
       resumeAt = Number(s.time) || 0;
       player.time = resumeAt;
       player.duration = (player.current?.duration ?? 0) / 1000;
@@ -288,6 +310,12 @@ function ensureAudio(): HTMLAudioElement {
   a.addEventListener('playing', () => {
     player.loading = false;
     errorStreak = 0;
+    const item = player.current;
+    if (failedVias.length && item) {
+      const other = player.via !== 'local' && player.via !== item.source;
+      toast(`${failedVias.map((v) => VIA_LABEL[v]).join('、')}无法播放，改用${VIA_LABEL[player.via]}${other ? '的同名歌曲' : ''}`);
+      failedVias = [];
+    }
   });
   a.addEventListener('canplay', () => (player.loading = false));
   a.addEventListener('seeked', updatePosition);
@@ -297,11 +325,26 @@ function ensureAudio(): HTMLAudioElement {
   return a;
 }
 
+const primaryVia = (item: PlayItem): Via => (item.local || !item.source ? 'local' : item.source);
+
 function onError() {
+  const item = player.current;
+  if (!item || !attached || !audio) return;
+  // 还有兜底音源：从出错的位置接着放
+  const fallback = item.alt?.[attempt];
+  if (fallback) {
+    attempt++;
+    failedVias.push(player.via);
+    const at = audio.currentTime || player.time;
+    resumeAt = at > 1 ? at : 0;
+    player.via = fallback.via;
+    player.loading = true;
+    audio.src = fallback.src;
+    audio.play().catch(() => {});
+    return;
+  }
   player.loading = false;
   player.playing = false;
-  const item = player.current;
-  if (!item || !attached) return;
   errorStreak++;
   // 连续失败时自动跳过，但不无限循环
   if (queue.value.length > 1 && errorStreak < Math.min(queue.value.length, 5)) {
@@ -330,6 +373,9 @@ function load(i: number, autoplay = true, keepResume = false) {
   const a = ensureAudio();
   if (!keepResume) resumeAt = 0;
   prefetched = '';
+  attempt = 0;
+  failedVias = [];
+  player.via = primaryVia(item);
   player.index = i;
   player.current = item;
   player.time = resumeAt;
@@ -527,6 +573,7 @@ export function stop() {
   player.playing = false;
   player.loading = false;
   player.panel = null;
+  player.full = false;
   player.context = '';
   lyric.value = [];
   if (media) media.metadata = null;