Add edit bookmark, offline cache, and unread filter

- Edit bookmark: full PATCH via swipe action or long-press context menu;
  EditBookmarkView covers URL, title, description, tags, and unread toggle
- Offline cache: bookmarks persisted to ApplicationSupport JSON and shown
  instantly on launch; cache is per-server, skipped when unread filter is on
- Unread filter: toolbar toggle sends ?unread=true to the API; title updates
  to "Unread"; no stale-cache flash when toggling
- Extract normalizedURL and parsedTags into String+Helpers, removing three
  copies of URL normalization and two of tag parsing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Marks/Models/Bookmark.swift

@@ -65,6 +65,20 @@ struct BookmarkCreate: Codable, Sendable {
     }
 }
 
+struct BookmarkUpdate: Codable, Sendable {
+    var url: String
+    var title: String
+    var description: String
+    var tagNames: [String]
+    var unread: Bool
+    var shared: Bool
+
+    enum CodingKeys: String, CodingKey {
+        case url, title, description, unread, shared
+        case tagNames = "tag_names"
+    }
+}
+
 struct SmartCollection: Identifiable, Sendable {
     let id = UUID()
     let name: String