feat(highlights): add relay indicator icon to highlight items

Add a small server icon at the bottom-left of each highlight that shows which relays the highlight was published to. The icon appears when publishedRelays information is available (for user-created highlights) and displays a tooltip with the list of relay URLs on hover.

- Import faServer icon from FontAwesome
- Add relay indicator to HighlightItem component
- Display formatted relay list in tooltip
- Add CSS styling for the indicator with hover effects
- Support both dark and light modes

src/index.css

@@ -1257,6 +1257,14 @@ body {
     color: #646cff;
   }
   
+  .highlight-relay-indicator {
+    color: #666;
+  }
+  
+  .highlight-relay-indicator:hover {
+    color: #333;
+  }
+  
   .highlight-text {
     color: #213547;
   }
@@ -1557,6 +1565,23 @@ body {
   font-size: 1.2rem;
   flex-shrink: 0;
   margin-top: 0.25rem;
+  position: relative;
+}
+
+.highlight-relay-indicator {
+  position: absolute;
+  bottom: -4px;
+  left: 0;
+  font-size: 0.7rem;
+  color: #888;
+  opacity: 0.7;
+  transition: opacity 0.2s ease;
+  cursor: help;
+}
+
+.highlight-relay-indicator:hover {
+  opacity: 1;
+  color: #aaa;
 }
 
 /* Level-colored quote icon */