diff --git a/server/public/admin.html b/server/public/admin.html
index b782f82..70f5df0 100644
--- a/server/public/admin.html
+++ b/server/public/admin.html
@@ -52,7 +52,8 @@
     }
     .crop-stage img {
       position: absolute; left: 0; top: 0;
-      max-width: none; cursor: grab;
+      max-width: none; max-height: none; width: auto; height: auto;
+      cursor: grab; will-change: left, top, width, height;
     }
     .crop-mask {
       position: absolute; left: 0; top: 0; pointer-events: none;
@@ -169,23 +170,26 @@
 
     function maskSize() {
       const stage = document.getElementById('cropStage')
-      const sw = stage.clientWidth
-      const sh = stage.clientHeight
-      let maskW = sw - 32
+      const sw = Math.max(stage.clientWidth, 1)
+      const sh = Math.max(stage.clientHeight, 1)
+      let maskW = Math.max(sw - 32, 1)
       let maskH = maskW / COVER_RATIO
       if (maskH > sh - 32) {
-        maskH = sh - 32
+        maskH = Math.max(sh - 32, 1)
         maskW = maskH * COVER_RATIO
       }
       return {
         maskW: maskW,
         maskH: maskH,
         maskLeft: (sw - maskW) / 2,
-        maskTop: (sh - maskH) / 2
+        maskTop: (sh - maskH) / 2,
+        stageW: sw,
+        stageH: sh
       }
     }
 
     function minCropScale() {
+      if (!cropState.naturalW || !cropState.naturalH) return 1
       const size = maskSize()
       return Math.max(size.maskW / cropState.naturalW, size.maskH / cropState.naturalH)
     }
@@ -194,40 +198,51 @@
       const stage = document.getElementById('cropStage')
       const img = document.getElementById('cropImage')
       const mask = document.getElementById('cropMask')
-      if (!stage || !img.naturalWidth || !cropState.naturalW) return
+      if (!stage || !cropState.naturalW || !cropState.scale) return
       const size = maskSize()
-      mask.style.right = 'auto'
-      mask.style.bottom = 'auto'
       mask.style.width = `${size.maskW}px`
       mask.style.height = `${size.maskH}px`
       mask.style.left = `${size.maskLeft}px`
       mask.style.top = `${size.maskTop}px`
 
-      img.style.width = `${cropState.naturalW * cropState.scale}px`
-      img.style.height = `${cropState.naturalH * cropState.scale}px`
-      img.style.transform = `translate(${cropState.x}px, ${cropState.y}px)`
+      const w = cropState.naturalW * cropState.scale
+      const h = cropState.naturalH * cropState.scale
+      img.style.width = `${w}px`
+      img.style.height = `${h}px`
+      img.style.left = `${cropState.x}px`
+      img.style.top = `${cropState.y}px`
+      img.style.transform = 'none'
     }
 
     function fitCropImage() {
-      const stage = document.getElementById('cropStage')
+      const size = maskSize()
+      // Modal must be visible first, otherwise stage size is 0 and scale collapses.
+      if (size.stageW < 40 || size.stageH < 40) return false
       const scale = minCropScale()
       cropState.scale = scale
-      cropState.x = (stage.clientWidth - cropState.naturalW * scale) / 2
-      cropState.y = (stage.clientHeight - cropState.naturalH * scale) / 2
+      cropState.x = (size.stageW - cropState.naturalW * scale) / 2
+      cropState.y = (size.stageH - cropState.naturalH * scale) / 2
       layoutCrop()
+      return true
     }
 
     function zoomCrop(factor) {
       if (!cropState.naturalW) return
       const size = maskSize()
+      if (size.stageW < 40 || size.stageH < 40) return
+      if (!cropState.scale || !Number.isFinite(cropState.scale)) {
+        fitCropImage()
+        return
+      }
+      const before = cropState.scale
+      const minScale = minCropScale()
+      const next = Math.min(Math.max(before * factor, minScale), minScale * 8)
+      if (!Number.isFinite(next) || Math.abs(next - before) < 0.0001) return
       const cx = size.maskLeft + size.maskW / 2
       const cy = size.maskTop + size.maskH / 2
-      const before = cropState.scale
-      const next = Math.min(Math.max(before * factor, minCropScale()), minCropScale() * 8)
-      if (Math.abs(next - before) < 0.0001) return
       // Keep the point under the crop center stable while zooming.
-      cropState.x = cx - ((cx - cropState.x) / before) * next
-      cropState.y = cy - ((cy - cropState.y) / before) * next
+      cropState.x = cx - ((cx - cropState.x) * next) / before
+      cropState.y = cy - ((cy - cropState.y) * next) / before
       cropState.scale = next
       layoutCrop()
     }
@@ -237,13 +252,20 @@
       const img = document.getElementById('cropImage')
       cropState.targetInput = targetInput
       cropState.hint = hint
+      cropState.scale = 0
+      cropState.x = 0
+      cropState.y = 0
       const url = URL.createObjectURL(file)
       img.onload = () => {
         cropState.naturalW = img.naturalWidth
         cropState.naturalH = img.naturalHeight
         URL.revokeObjectURL(url)
-        fitCropImage()
         modal.classList.add('on')
+        requestAnimationFrame(() => {
+          if (!fitCropImage()) {
+            setTimeout(() => fitCropImage(), 50)
+          }
+        })
       }
       img.src = url
     }
@@ -252,20 +274,16 @@
       document.getElementById('cropModal').classList.remove('on')
       document.getElementById('cropImage').removeAttribute('src')
       cropState.targetInput = null
+      cropState.scale = 0
     }
 
     function exportCroppedFile() {
-      const stage = document.getElementById('cropStage')
-      const mask = document.getElementById('cropMask')
       const img = document.getElementById('cropImage')
-      const maskRect = mask.getBoundingClientRect()
-      const stageRect = stage.getBoundingClientRect()
-      const maskLeft = maskRect.left - stageRect.left
-      const maskTop = maskRect.top - stageRect.top
-      const sx = (maskLeft - cropState.x) / cropState.scale
-      const sy = (maskTop - cropState.y) / cropState.scale
-      const sw = maskRect.width / cropState.scale
-      const sh = maskRect.height / cropState.scale
+      const size = maskSize()
+      const sx = (size.maskLeft - cropState.x) / cropState.scale
+      const sy = (size.maskTop - cropState.y) / cropState.scale
+      const sw = size.maskW / cropState.scale
+      const sh = size.maskH / cropState.scale
       const outW = 1500
       const outH = Math.round(outW / COVER_RATIO)
       const canvas = document.createElement('canvas')
@@ -275,8 +293,12 @@
       ctx.fillStyle = '#111827'
       ctx.fillRect(0, 0, outW, outH)
       ctx.drawImage(img, sx, sy, sw, sh, 0, 0, outW, outH)
-      return new Promise((resolve) => {
+      return new Promise((resolve, reject) => {
         canvas.toBlob((blob) => {
+          if (!blob) {
+            reject(new Error('裁剪失败'))
+            return
+          }
           resolve(new File([blob], `cover-${Date.now()}.jpg`, { type: 'image/jpeg' }))
         }, 'image/jpeg', 0.92)
       })