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/String+Helpers.swift

@@ -0,0 +1,14 @@
+import Foundation
+
+extension String {
+    var normalizedURL: String {
+        let trimmed = trimmingCharacters(in: .whitespacesAndNewlines)
+        return (trimmed.hasPrefix("http://") || trimmed.hasPrefix("https://")) ? trimmed : "https://" + trimmed
+    }
+
+    var parsedTags: [String] {
+        split(separator: ",")
+            .map { $0.trimmingCharacters(in: .whitespaces) }
+            .filter { !$0.isEmpty }
+    }
+}