fix(drone): fix main panel height collapse in flex output container

Replace height:100% with flex:1+min-height:0 on .drone-visuals-container
so it fills the flex-column .output-panel correctly (height:100% collapses
inside a scroll container). Add min-height:400px to .drone-main-map so
Leaflet has pixel dimensions to render into.

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

static/css/modes/drone.css

@@ -4,8 +4,8 @@
 .drone-visuals-container {
     display: none;
     flex-direction: column;
-    width: 100%;
-    height: 100%;
+    flex: 1;
+    min-height: 0;
     overflow: hidden;
     background: var(--bg-primary);
     padding: 0;
@@ -67,6 +67,7 @@
 .drone-main-map {
     flex: 1;
     min-width: 0;
+    min-height: 400px;
 }
 
 .drone-empty-state {