style: add blur-up progressive image loading styles

input.css

@@ -727,3 +727,43 @@
     animation: none;
   }
 }
+
+/* ========== Blur-up 渐进图片加载 ========== */
+.blur-img {
+  position: relative;
+  display: block;
+  overflow: hidden;
+  aspect-ratio: var(--ar);
+  /* 加载中灰底（占位图未加载完时） */
+  background: var(--color-paper-code-bg, #f5f5f5);
+  border-radius: 6px;
+  margin: 1em 0;
+}
+.blur-img-placeholder {
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+  /* 20px 占位图放大后模糊，scale 遮边缘 */
+  filter: blur(20px) saturate(1.2);
+  transform: scale(1.1);
+}
+.blur-img-full {
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  object-fit: cover;
+  opacity: 0;
+  transition: opacity 0.4s ease;
+  z-index: 1;
+}
+.blur-img-full.is-loaded {
+  opacity: 1;
+}
+
+/* 暗色模式灰底 */
+.dark .blur-img {
+  background: var(--color-paper-code-bg, #2a2a2a);
+}