Improve aircraft icons and add weight-based color coding

- Replaced simple geometric shapes with detailed, realistic SVG icons for all aircraft types
- Implemented color differentiation based on aircraft weight categories:
  * Light (<7000kg): Sky blue
  * Medium (7000-34000kg): Green
  * Large (34000-136000kg): Orange
  * Heavy (>136000kg): Red
- Created new icons for: commercial airliner, helicopter, cargo aircraft, general aviation, military fighter, ground vehicle
- Updated legend to reflect new weight-based categories
- Modified getAircraftColor() to assign colors based on aircraft category/weight

Closes #17

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

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

assets/static/icons/ground.svg

@@ -1,10 +1,24 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <svg width="32" height="32" viewBox="0 0 32 32" xmlns="http://www.w3.org/2000/svg">
   <g transform="translate(16,16)">
-    <!-- Ground vehicle - truck/car shape -->
-    <rect x="-6" y="-4" width="12" height="8" fill="currentColor" rx="2"/>
+    <!-- Airport ground service vehicle -->
+    <!-- Main body -->
+    <path d="M-6,-3 L-6,3 L6,3 L6,-1 L4,-3 Z" fill="currentColor"/>
+    <!-- Cab/cockpit -->
+    <path d="M4,-3 L4,1 L6,1 L6,-1 Z" fill="currentColor" opacity="0.8"/>
+    <!-- Windows -->
+    <rect x="4.5" y="-2.5" width="1" height="1.5" fill="currentColor" opacity="0.5"/>
+    <!-- Cargo area -->
+    <rect x="-5" y="-2" width="7" height="3" fill="none" stroke="currentColor" stroke-width="0.3" opacity="0.5"/>
     <!-- Wheels -->
-    <circle cx="-3" cy="2" r="2" fill="#333"/>
-    <circle cx="3" cy="2" r="2" fill="#333"/>
+    <circle cx="-4" cy="4" r="1.5" fill="currentColor" opacity="0.7"/>
+    <circle cx="-1" cy="4" r="1.5" fill="currentColor" opacity="0.7"/>
+    <circle cx="4" cy="4" r="1.5" fill="currentColor" opacity="0.7"/>
+    <!-- Wheel details -->
+    <circle cx="-4" cy="4" r="0.5" fill="currentColor" opacity="0.4"/>
+    <circle cx="-1" cy="4" r="0.5" fill="currentColor" opacity="0.4"/>
+    <circle cx="4" cy="4" r="0.5" fill="currentColor" opacity="0.4"/>
+    <!-- Beacon light on top -->
+    <rect x="-1" y="-4" width="2" height="1" fill="currentColor" opacity="0.6"/>
   </g>
 </svg>