fix(SharePoster): 严格裁剪封面图避免溢出到信息区

修改封面图裁剪逻辑，根据源图与目标区域的纵横比选择水平或垂直裁剪
添加封面区域样式限制，防止内容溢出

@@ -365,15 +365,35 @@ async function compose_poster() {
         rounded_rect_path(ctx, card_x, card_y, card_w, card_h, card_radius)
         rounded_rect_path(ctx, card_x, card_y, card_w, card_h, card_radius)
         ctx.clip()
         ctx.clip()
 
 
-        // 封面图（对象填充）
+        // 封面图（严格裁剪到封面区域，避免溢出到信息区）
         const cover_img = await load_image(cover_src.value)
         const cover_img = await load_image(cover_src.value)
         if (cover_img) {
         if (cover_img) {
-            const scale = Math.max(card_w / cover_img.width, cover_h / cover_img.height)
-            const dw = cover_img.width * scale
-            const dh = cover_img.height * scale
-            const dx = card_x + (card_w - dw) / 2
-            const dy = card_y + (cover_h - dh) / 2
-            ctx.drawImage(cover_img, dx, dy, dw, dh)
+            const sw = cover_img.width
+            const sh = cover_img.height
+            const dest_ar = card_w / cover_h
+            const src_ar = sw / sh
+            let sx = 0, sy = 0, s_w = sw, s_h = sh
+            // 根据源图与目标区域的纵横比，选择水平或垂直裁剪
+            if (src_ar > dest_ar) {
+                // 源图更宽：水平裁剪
+                s_h = sh
+                s_w = Math.round(sh * dest_ar)
+                sx = Math.round((sw - s_w) / 2)
+                sy = 0
+            } else {
+                // 源图更窄或更高：垂直裁剪
+                s_w = sw
+                s_h = Math.round(sw / dest_ar)
+                sx = 0
+                sy = Math.round((sh - s_h) / 2)
+            }
+            // 目标区域严格限定在封面矩形
+            ctx.save()
+            ctx.beginPath()
+            ctx.rect(card_x, card_y, card_w, cover_h)
+            ctx.clip()
+            ctx.drawImage(cover_img, sx, sy, s_w, s_h, card_x, card_y, card_w, cover_h)
+            ctx.restore()
         } else {
         } else {
             ctx.fillStyle = '#f3f4f6' // gray-100
             ctx.fillStyle = '#f3f4f6' // gray-100
             ctx.fillRect(card_x, card_y, card_w, cover_h)
             ctx.fillRect(card_x, card_y, card_w, cover_h)
@@ -490,6 +510,12 @@ watch(() => [props.course, props.qr_url], () => {
             object-fit: contain;
             object-fit: contain;
             display: block;
             display: block;
         }
         }
+        .PosterCover {
+            // 固定封面高度为卡片宽度的 2/3，避免溢出到信息区
+            aspect-ratio: 3 / 2;
+            width: 100%;
+            overflow: hidden;
+        }
         .PosterInfo {
         .PosterInfo {
             overflow: hidden;
             overflow: hidden;
             .PosterQR {
             .PosterQR {