feat: UI improvements and Space category

- Add new "Space" category with Satellite and ISS SSTV modes
- Rename "Scanner" to "Listening Post"
- SSTV now uses global SDR device selector
- Meshtastic map markers more visible (stronger glow, larger size)
- CSS layout fixes using flex instead of fixed heights

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/js/modes/meshtastic.js

@@ -625,9 +625,9 @@ const Meshtastic = (function() {
         const icon = L.divIcon({
             className: 'mesh-marker-wrapper',
             html: `<div class="${markerClass}">${shortName.slice(0, 2).toUpperCase()}</div>`,
-            iconSize: [28, 28],
-            iconAnchor: [14, 14],
-            popupAnchor: [0, -14]
+            iconSize: [32, 32],
+            iconAnchor: [16, 16],
+            popupAnchor: [0, -16]
         });
 
         // Build telemetry section