fix(build): SSR 安全化 PlaylistCardGenerator canvasHeight 避免 Nitro 构建 OOM(exit 137)

上版在 canvasHeight computed 里对每首歌 document.createElement('canvas')+getContext('2d')，Nitro prerender/rollup bundler 在 Node 环境下要么 document undefined 抛错 fallback，要么 prerender 时对大歌单反复创建 2D context 撑爆虚拟内存 -> 5min 后 OOM Killer(exit 137)。

修复：
- 新增 isBrowser = typeof document !== 'undefined' 判断
- getSharedCtx(): 浏览器环境才创建 1 个共享 2D context 并跨次复用（不再每首歌 createElement）
- fitText(ctx, ...) 抽成双分支：有 ctx 用 measureText（客户端精确），无 ctx 用 CJK>0x2e80 Unicode 判断+经验字符宽度（16px 中文 1.05em/英文 0.55em）估算换行，保守估算高度
- 同时 fitText 支持文本中带 \\n 换行手动切分
- canvasHeight 中不再 new canvas/context，SSR 环境也能安全执行+算出估算高度

app/components/Admin/PlaylistCardGenerator.vue

@@ -91,30 +91,69 @@ const songLeftX = 44
 const songRightX = 360
 const songAreaWidth = songRightX - songLeftX
 
+const isBrowser = typeof document !== 'undefined'
+
+let _sharedCtx = null
+const getSharedCtx = () => {
+  if (!isBrowser) return null
+  if (_sharedCtx) return _sharedCtx
+  try {
+    const c = document.createElement('canvas')
+    _sharedCtx = c.getContext('2d')
+  } catch { _sharedCtx = null }
+  return _sharedCtx
+}
+
 const fitText = (ctx, text, fontSize, maxWidth, lineHeight) => {
-  ctx.font = `${fontSize}px "Inter", "system-ui", "PingFang SC", "Microsoft YaHei", sans-serif`
+  const hasCtx = !!ctx
-  const chars = Array.from(text)
+  const fontFamily = `"Inter", "system-ui", "PingFang SC", "Microsoft YaHei", sans-serif`
+  const avgCharWidth = fontSize * 0.55
+  const cjkCharWidth = fontSize * 1.05
+  const splitLines = String(text || '').split(/\r?\n/)
   const lines = []
-  let current = ''
+  for (const rawLine of splitLines) {
-  for (const ch of chars) {
+    if (hasCtx) {
-    const tryStr = current + ch
+      ctx.font = `${fontSize}px ${fontFamily}`
-    const w = ctx.measureText(tryStr).width
+      const chars = Array.from(rawLine)
-    if (w > maxWidth && current) {
+      let current = ''
-      lines.push(current)
+      for (const ch of chars) {
-      current = ch
+        const tryStr = current + ch
+        const w = ctx.measureText(tryStr).width
+        if (w > maxWidth && current) {
+          lines.push(current)
+          current = ch
+        } else {
+          current = tryStr
+        }
+      }
+      if (current) lines.push(current)
     } else {
-      current = tryStr
+      const chars = Array.from(rawLine)
+      let current = ''
+      let curW = 0
+      for (const ch of chars) {
+        const code = ch.codePointAt(0) || 0
+        const isCJK = code > 0x2e80
+        const w = isCJK ? cjkCharWidth : avgCharWidth
+        if (curW + w > maxWidth && current) {
+          lines.push(current)
+          current = ch
+          curW = w
+        } else {
+          current += ch
+          curW += w
+        }
+      }
+      if (current) lines.push(current)
     }
   }
-  if (current) lines.push(current)
   return { lines, lineHeight }
 }
 
 const canvasHeight = computed(() => {
   const count = Math.min(props.songs.length, maxDisplaySongs)
   if (!count) return 600
-  const canvas = document.createElement('canvas')
+  const ctx = getSharedCtx()
-  const ctx = canvas.getContext('2d')
   let totalH = 0
   const maxWidthTitle = songAreaWidth
   const maxWidthArtist = Math.max(100, songAreaWidth - 10)