feat(eventFirmware): point DEF CON 34 at its branding icon

Author: jamesarich

data/eventFirmware.json

@@ -106,7 +106,7 @@
       "eventEnd": "2026-08-09",
       "timeZone": "America/Los_Angeles",
       "location": "Las Vegas Convention Center, Las Vegas, Nevada, USA",
-      "iconUrl": null,
+      "iconUrl": "https://api.meshtastic.org/resource/eventFirmware/defcon34.png",
       "accentColor": "#0D294A",
       "domain": "defcon.meshtastic.org",
       "links": [