fix: 图片上传自动压缩增强

- 渐进式缩小：最多5轮，每轮缩小30%+降低JPEG质量
- 确保最终一定在大小限制内
- QR/logo最大500KB/800px，背景最大1MB/1200px

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/MyDiary.vue

@@ -504,36 +504,38 @@ function readFileAsBase64(file) {
   })
 }
 
-// Compress image if too large (keeps PNG for small images, JPEG for large)
-function compressImage(base64, maxSize = 500000) {
+// Compress image if too large
+function compressImage(base64, maxSize = 500000, maxDim = 800) {
   return new Promise((resolve) => {
     if (base64.length <= maxSize) { resolve(base64); return }
     const img = new Image()
     img.onload = () => {
       const canvas = document.createElement('canvas')
       let w = img.width, h = img.height
-      const maxDim = 600
+      // Shrink progressively until it fits
+      let scale = 1
       if (w > maxDim || h > maxDim) {
-        const ratio = Math.min(maxDim / w, maxDim / h)
-        w = Math.round(w * ratio)
-        h = Math.round(h * ratio)
+        scale = Math.min(maxDim / w, maxDim / h)
       }
-      canvas.width = w
-      canvas.height = h
-      canvas.getContext('2d').drawImage(img, 0, 0, w, h)
-      // Try PNG first, then JPEG with decreasing quality
-      let result = canvas.toDataURL('image/png')
-      if (result.length > maxSize) {
-        let quality = 0.85
-        while (quality > 0.2) {
+      for (let attempt = 0; attempt < 5; attempt++) {
+        const cw = Math.round(w * scale)
+        const ch = Math.round(h * scale)
+        canvas.width = cw
+        canvas.height = ch
+        canvas.getContext('2d').drawImage(img, 0, 0, cw, ch)
+        let quality = 0.8
+        let result = canvas.toDataURL('image/jpeg', quality)
+        while (result.length > maxSize && quality > 0.2) {
+          quality -= 0.15
           result = canvas.toDataURL('image/jpeg', quality)
-          if (result.length <= maxSize) break
-          quality -= 0.1
         }
+        if (result.length <= maxSize) { resolve(result); return }
+        scale *= 0.7 // shrink more
       }
-      resolve(result)
+      // Last resort
+      resolve(canvas.toDataURL('image/jpeg', 0.3))
     }
-    img.onerror = () => resolve(base64) // fallback: return original
+    img.onerror = () => resolve(base64)
     img.src = base64
   })
 }
@@ -589,7 +591,8 @@ async function handleUpload(type, event) {
     }
 
     const maxSize = type === 'bg' ? 1000000 : 500000
-    base64 = await compressImage(base64, maxSize)
+    const maxDim = type === 'bg' ? 1200 : 800
+    base64 = await compressImage(base64, maxSize, maxDim)
     const fieldMap = { logo: 'brand_logo', bg: 'brand_bg', qr: 'qr_code' }
     const field = fieldMap[type]
     if (!field) return