Add balloon, glider, and UAV sprites with admin sprite management

New dedicated pixel art sprites for ADS-B emitter categories B2
(lighter-than-air → balloon), B1 (glider/sailplane → glider), and
B6 (UAV → uav), replacing the previous fallback to smallProp. Adds
an admin Sprites tab for previewing and uploading replacement PNGs
for all 9 aircraft sprite types.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ads-bit.js

@@ -99,7 +99,10 @@ class ADSBit {
             narrowBody: new Image(),
             wideBody: new Image(),
             heavy: new Image(),
-            helicopter: new Image()
+            helicopter: new Image(),
+            balloon: new Image(),
+            glider: new Image(),
+            uav: new Image()
         };
 
         // Track which images have loaded
@@ -109,7 +112,10 @@ class ADSBit {
             narrowBody: false,
             wideBody: false,
             heavy: false,
-            helicopter: false
+            helicopter: false,
+            balloon: false,
+            glider: false,
+            uav: false
         };
 
         // Load all aircraft images
@@ -785,10 +791,10 @@ class ADSBit {
                 break;
             case 'A6': type = 'narrowBody'; break;     // High performance (> 5g, > 400 kts)
             case 'A7': type = 'helicopter'; break;     // Rotorcraft
-            case 'B1': type = 'smallProp'; break;      // Glider / sailplane
-            case 'B2': type = 'smallProp'; break;      // Lighter-than-air
+            case 'B1': type = 'glider'; break;         // Glider / sailplane
+            case 'B2': type = 'balloon'; break;        // Lighter-than-air
             case 'B4': type = 'smallProp'; break;      // Skydiver drop plane
-            case 'B6': type = 'smallProp'; break;      // UAV
+            case 'B6': type = 'uav'; break;            // UAV
             default:   type = 'narrowBody'; break;     // Unknown or surface vehicles
         }