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/MarksApp.swift

@@ -33,7 +33,7 @@ struct MainContainer: View {
         self.onDisconnect = onDisconnect
         let api = LinkdingAPI(config: config)
         let claude = ClaudeService.load()
-        _viewModel = State(wrappedValue: BookmarksViewModel(api: api, claude: claude))
+        _viewModel = State(wrappedValue: BookmarksViewModel(api: api, claude: claude, cacheKey: config.host))
     }
 
     var body: some View {