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/Views/AddBookmarkView.swift

@@ -64,15 +64,12 @@ struct AddBookmarkView: View {
     }
 
     private func save() {
-        var urlString = url.trimmingCharacters(in: .whitespaces)
-        if !urlString.hasPrefix("http://") && !urlString.hasPrefix("https://") {
-            urlString = "https://" + urlString
-        }
+        let urlString = url.normalizedURL
         guard URL(string: urlString) != nil else {
             error = "Invalid URL"
             return
         }
-        let tags = tagsText.split(separator: ",").map { $0.trimmingCharacters(in: .whitespaces) }.filter { !$0.isEmpty }
+        let tags = tagsText.parsedTags
         isSaving = true
         error = nil
         Task {