feat: add private bookmarks support with NIP-51 and visual indicators

- Updated bookmark types to support private bookmarks with isPrivate and encryptedContent fields
- Enhanced bookmark service to detect encrypted content and mark bookmarks as private
- Added visual indicators for private bookmarks with lock icon and special styling
- Added CSS styles for private bookmarks with red accent border and gradient background
- Updated BookmarkItem component to show private bookmark indicators
- Maintained compatibility with existing public bookmark functionality

src/index.css

@@ -381,6 +381,23 @@ body {
   font-family: monospace;
 }
 
+/* Private Bookmark Styles */
+.private-bookmark {
+  border-left: 4px solid #ff6b6b;
+  background: linear-gradient(135deg, #2a2a2a 0%, #1f1f1f 100%);
+}
+
+.private-bookmark:hover {
+  border-color: #ff6b6b;
+  box-shadow: 0 2px 8px rgba(255, 107, 107, 0.2);
+}
+
+.private-indicator {
+  margin-left: 0.5rem;
+  font-size: 0.9rem;
+  color: #ff6b6b;
+}
+
 @media (prefers-color-scheme: light) {
   :root {
     color: #213547;
@@ -435,4 +452,14 @@ body {
     background: #e9ecef;
     color: #666;
   }
+  
+  .private-bookmark {
+    border-left-color: #ff6b6b;
+    background: linear-gradient(135deg, #f5f5f5 0%, #e9ecef 100%);
+  }
+  
+  .private-bookmark:hover {
+    border-color: #ff6b6b;
+    box-shadow: 0 2px 8px rgba(255, 107, 107, 0.2);
+  }
 }