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/commercial.svg

@@ -1,7 +1,21 @@
 <?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)">
-    <!-- Standard commercial aircraft shape -->
-    <path d="M0,-12 L-8,8 L-2,8 L0,12 L2,8 L8,8 Z" fill="currentColor"/>
+    <!-- Commercial airliner with more realistic proportions -->
+    <!-- Fuselage -->
+    <path d="M0,-14 L-1.5,-12 L-1.5,7 L-1,10 L0,11 L1,10 L1.5,7 L1.5,-12 Z" fill="currentColor"/>
+    <!-- Main wings -->
+    <path d="M-1,0 L-12,3 L-12,5 L-1,3 Z" fill="currentColor"/>
+    <path d="M1,0 L12,3 L12,5 L1,3 Z" fill="currentColor"/>
+    <!-- Tail wings (horizontal stabilizers) -->
+    <path d="M-0.8,8 L-5,9.5 L-5,10.5 L-0.8,9.5 Z" fill="currentColor"/>
+    <path d="M0.8,8 L5,9.5 L5,10.5 L0.8,9.5 Z" fill="currentColor"/>
+    <!-- Vertical stabilizer -->
+    <path d="M0,6 L-0.5,6 L-2,11 L0,11 L2,11 L0.5,6 Z" fill="currentColor"/>
+    <!-- Nose cone -->
+    <ellipse cx="0" cy="-13.5" rx="1.5" ry="2" fill="currentColor"/>
+    <!-- Engine nacelles -->
+    <ellipse cx="-4" cy="2" rx="1" ry="2" fill="currentColor"/>
+    <ellipse cx="4" cy="2" rx="1" ry="2" fill="currentColor"/>
   </g>
 </svg>