feat: list mode 50/page, card mode left-image rows, AI cover generation command

- blog_paginate: 纯列表模式每页 50 条（list_per_page 可调），其余模式仍走 per_page
- 图文(card)模式：卡片网格改为左图右文横排，小屏回退竖排；摘要放宽到 100 字
- 新增 covers:generate：复用 AI 设置的 LLM 配置调 /image_generation（默认 image-01），
  为无封面文章批量生成封面存入 media cover 集合；支持 --all/--limit/--post/--style/--dry-run
- AI 设置页新增图片模型、封面风格两项配置
- 修正 HomeViewModeTest：列表模式为纯标题（不再显示摘要），并断言每页 50 条与图文布局

public/themes/sablog/style.css

@@ -122,24 +122,19 @@ a:hover {
     margin-left: 10px;
 }
 /* 图文模式：卡片网格 */
-.post-list-card {
-    display: grid;
-    grid-template-columns: repeat(3, 1fr);
-    gap: 16px;
-    margin-bottom: 16px;
-}
 .post-list-card .post-card {
-    margin-bottom: 0;
-    padding: 0;
-    overflow: hidden;
     display: flex;
-    flex-direction: column;
+    gap: 16px;
+    align-items: stretch;
 }
 .post-list-card .post-cover {
-    display: block;
+    flex: 0 0 200px;
+    width: 200px;
     aspect-ratio: 16 / 10;
+    margin: 0;
     background: #eef2f5;
     overflow: hidden;
+    border-radius: 4px;
 }
 .post-list-card .post-cover img {
     width: 100%;
@@ -159,11 +154,12 @@ a:hover {
     background: linear-gradient(135deg, #eef2f5 0%, #dde7ee 100%);
 }
 .post-list-card .post-card-body {
-    padding: 12px 14px;
     flex: 1;
+    min-width: 0;
+    padding: 2px 0;
 }
 .post-list-card .post-title {
-    font-size: 15px;
+    font-size: 16px;
     margin-bottom: 6px;
 }
 .post-list-card .post-excerpt {
@@ -175,14 +171,13 @@ a:hover {
     -webkit-box-orient: vertical;
     overflow: hidden;
 }
-@media (max-width: 1024px) {
-    .post-list-card {
-        grid-template-columns: repeat(2, 1fr);
-    }
-}
 @media (max-width: 640px) {
-    .post-list-card {
-        grid-template-columns: 1fr;
+    .post-list-card .post-card {
+        flex-direction: column;
+    }
+    .post-list-card .post-cover {
+        flex: none;
+        width: 100%;
     }
 }