feat: topbar legacy-sync button, card view mode with cover fallback, year/month/day archives, richer page cache admin, stylevar repeater editor

- topbar: Livewire LegacySyncButton next to 写文章 (moved from user menu)
- view modes: add 图文(card) grid with cover fallback chain (cover → attachment → first content image → placeholder); list mode now title-only compact
- archives: year/month/day index grid
- page cache admin: size + expiry columns, total size, cache policy note
- theme inject: stylevars switch from KeyValue to Repeater with multi-line HTML editing
- import: html_entity_decode for category/post-title/link/keywords/comment author

app/Models/Post.php

@@ -133,6 +133,35 @@ class Post extends Model implements HasMedia
         return $cover->getUrl($conversion) ?: $cover->getUrl();
     }
 
+    /**
+     * 封面回退链：显式封面 → 附件/正文里的第一张图。
+     * 都没有时返回 null，由调用方决定兜底（占位图或不显示）。
+     */
+    public function coverOrFirstImageUrl(): ?string
+    {
+        if ($url = $this->coverImageUrl()) {
+            return $url;
+        }
+
+        // 附件里的第一张图片
+        $attachment = $this->getMedia('attachments')
+            ->first(fn ($m) => str_starts_with((string) $m->mime_type, 'image/'));
+        if ($attachment) {
+            return $attachment->getUrl();
+        }
+
+        // 正文 HTML/Markdown 里的第一张图片地址
+        $content = (string) $this->content;
+        if (preg_match('/<img[^>]+src=["\']([^"\']+)["\']/i', $content, $m)) {
+            return $m[1];
+        }
+        if (preg_match('/!\[[^\]]*\]\((https?:\/\/[^)\s]+)[^)]*\)/i', $content, $m)) {
+            return $m[1];
+        }
+
+        return null;
+    }
+
     public function comments(): HasMany
     {
         return $this->hasMany(Comment::class);