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/OnboardingView.swift

@@ -125,10 +125,7 @@ struct OnboardingView: View {
     }
 
     private func parseConfig() throws -> ServerConfig {
-        var raw = urlText.trimmingCharacters(in: .whitespacesAndNewlines)
-        if !raw.hasPrefix("http://") && !raw.hasPrefix("https://") {
-            raw = "https://" + raw
-        }
+        let raw = urlText.normalizedURL
         guard let comps = URLComponents(string: raw),
               let host = comps.host, !host.isEmpty else {
             throw URLError(.badURL)