Fix white-on-white text visibility in aircraft popups

Override Leaflet's default popup styles with !important declarations to ensure
proper text visibility in aircraft information popups:
- Set dark background (#2d2d2d) for popup content wrapper and tip
- Force white text color (#ffffff) for all popup text elements
- Ensure labels remain muted gray (#888) for visual hierarchy
- Preserve accent colors for flight ID (blue) and callsign (green)

The issue was caused by Leaflet CSS overriding custom popup styling,
resulting in poor readability with white text on white backgrounds.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

assets/static/css/style.css

@@ -362,20 +362,39 @@ body {
     z-index: 1000;
 }
 
+/* Leaflet popup override - ensure our styles take precedence */
+.leaflet-popup-content-wrapper {
+    background: #2d2d2d !important;
+    color: #ffffff !important;
+    border-radius: 8px;
+}
+
+.leaflet-popup-content {
+    margin: 12px !important;
+    color: #ffffff !important;
+}
+
+.leaflet-popup-tip {
+    background: #2d2d2d !important;
+}
+
 .aircraft-popup {
     min-width: 300px;
     max-width: 400px;
+    color: #ffffff !important;
 }
 
 .popup-header {
     border-bottom: 1px solid #404040;
     padding-bottom: 0.5rem;
     margin-bottom: 0.75rem;
+    color: #ffffff !important;
 }
 
 .flight-info {
     font-size: 1.1rem;
     font-weight: bold;
+    color: #ffffff !important;
 }
 
 .icao-flag {
@@ -384,21 +403,27 @@ body {
 }
 
 .flight-id {
-    color: #00a8ff;
+    color: #00a8ff !important;
     font-family: monospace;
 }
 
 .callsign {
-    color: #00ff88;
+    color: #00ff88 !important;
 }
 
 .popup-details {
     font-size: 0.9rem;
+    color: #ffffff !important;
 }
 
 .detail-row {
     margin-bottom: 0.5rem;
     padding: 0.25rem 0;
+    color: #ffffff !important;
+}
+
+.detail-row strong {
+    color: #ffffff !important;
 }
 
 .detail-grid {
@@ -415,13 +440,13 @@ body {
 
 .detail-item .label {
     font-size: 0.8rem;
-    color: #888;
+    color: #888 !important;
     margin-bottom: 0.1rem;
 }
 
 .detail-item .value {
     font-weight: bold;
-    color: #ffffff;
+    color: #ffffff !important;
 }
 
 @media (max-width: 768px) {