piano-sheet(upload): mobile/pad first，主入口直调后置摄像头

- 主 CTA「拍下一页」+ capture=environment，副 CTA「从相册」multiple
- 底部 sticky 上传按钮，iOS safe area 兜住刘海/Home indicator
- 客户端压缩 1800px JPEG 0.85（createImageBitmap + EXIF 自动旋转）
- 页码 ↑↓ 移动 + ✕ 删除替代旧拖拽

apps/piano-sheet/frontend/src/lib/compress.js

@@ -0,0 +1,32 @@
+// 移动端拍照原图常 5–12MB，1500–1800px 长边 + JPEG 0.85 已经够看清楚音符。
+// 用 createImageBitmap 的 imageOrientation: 'from-image' 自动按 EXIF 旋转，
+// 否则横拍照在 canvas 里会变成竖图。
+
+export async function compressImage(file, { maxEdge = 1800, quality = 0.85 } = {}) {
+  if (!file || !file.type || !file.type.startsWith('image/')) return file
+
+  let bitmap
+  try {
+    bitmap = await createImageBitmap(file, { imageOrientation: 'from-image' })
+  } catch {
+    return file
+  }
+
+  const { width, height } = bitmap
+  const scale = Math.min(1, maxEdge / Math.max(width, height))
+  const w = Math.max(1, Math.round(width * scale))
+  const h = Math.max(1, Math.round(height * scale))
+
+  const canvas = document.createElement('canvas')
+  canvas.width = w
+  canvas.height = h
+  const ctx = canvas.getContext('2d')
+  ctx.drawImage(bitmap, 0, 0, w, h)
+  bitmap.close?.()
+
+  const blob = await new Promise((res) => canvas.toBlob(res, 'image/jpeg', quality))
+  if (!blob || blob.size >= file.size) return file
+
+  const baseName = file.name?.replace(/\.[^.]+$/, '') || 'photo'
+  return new File([blob], `${baseName}.jpg`, { type: 'image/jpeg', lastModified: Date.now() })
+}