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>

admin/admin.html

@@ -40,6 +40,7 @@
             <button class="tab" data-tab="location">LOCATION</button>
             <button class="tab" data-tab="display">DISPLAY</button>
             <button class="tab" data-tab="tuning">TUNING</button>
+            <button class="tab" data-tab="sprites">SPRITES</button>
             <button class="tab" data-tab="security">SECURITY</button>
         </nav>
 
@@ -201,6 +202,20 @@
                 </div>
             </section>
 
+            <!-- Sprites Tab -->
+            <section id="tab-sprites" class="tab-pane hidden">
+                <h2>Aircraft Sprites</h2>
+                <div class="sprite-info">
+                    <h3>Sprite Specifications</h3>
+                    <p><strong>Size:</strong> 500 x 333 px &nbsp; <strong>Format:</strong> PNG 8-bit RGBA (transparent background)</p>
+                    <p><strong>Orientation:</strong> Facing right (auto-flipped for westbound aircraft)</p>
+                    <p><strong>Style:</strong> Retro SNES pixel art, white/blue/red color scheme</p>
+                </div>
+                <div id="sprite-grid" class="sprite-grid">
+                    <!-- Populated by JS -->
+                </div>
+            </section>
+
             <!-- Security Tab -->
             <section id="tab-security" class="tab-pane hidden">
                 <h2>Change Admin Password</h2>