feat(BookmarkRow): show a content excerpt under each bookmark title

Surface a bit of the page content in the list without fetching/parsing pages
ourselves — linkding already returns the scraped metadata in the bookmark
payload. Add Bookmark.contentExcerpt (website_description, falling back to the
user's note) and render it under the title: AI summary takes priority and stays
italic, otherwise the scraped excerpt shows. Rows with neither degrade to the
existing favicon + title layout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Marks/Views/BookmarkRow.swift

@@ -55,10 +55,10 @@ struct BookmarkRow: View {
                 }
             }
 
-            if let summary = bookmark.aiSummary {
-                Text(summary)
+            if let excerpt = rowExcerpt {
+                Text(excerpt.text)
                     .font(.subheadline)
-                    .italic()
+                    .italic(excerpt.isAI)
                     .foregroundStyle(.secondary)
                     .lineLimit(2)
                     .padding(.leading, 44)
@@ -106,6 +106,18 @@ struct BookmarkRow: View {
         }
     }
 
+    /// Excerpt shown under the title: prefer the AI summary (italic), else the
+    /// page's scraped description / user note. nil hides the line entirely.
+    private var rowExcerpt: (text: String, isAI: Bool)? {
+        if let s = bookmark.aiSummary?.trimmingCharacters(in: .whitespacesAndNewlines), !s.isEmpty {
+            return (s, true)
+        }
+        if let e = bookmark.contentExcerpt {
+            return (e, false)
+        }
+        return nil
+    }
+
     private var effectiveTags: [String] {
         let base = bookmark.tagNames
         let ai = bookmark.aiTags ?? []