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/Models/Bookmark.swift

@@ -41,6 +41,17 @@ struct Bookmark: Codable, Identifiable, Sendable, Hashable {
     var domain: String {
         URL(string: url)?.host ?? url
     }
+
+    /// A short content snippet for list previews: the page's scraped meta
+    /// description, falling back to the user's own note. nil when neither exists.
+    var contentExcerpt: String? {
+        for candidate in [websiteDescription, description] {
+            if let c = candidate?.trimmingCharacters(in: .whitespacesAndNewlines), !c.isEmpty {
+                return c
+            }
+        }
+        return nil
+    }
 }
 
 struct BookmarkResponse: Codable, Sendable {