Fix exported plan GPX missing overnight waypoint metadata

ExportButton.getWaypoints() wasn't reading the overnight flag from
Yjs Y.Maps, so exported plan GPX was missing <type>overnight</type>.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/planner/app/components/ExportButton.tsx

@@ -23,6 +23,7 @@ function getWaypoints(yjs: YjsState) {
     lat: yMap.get("lat") as number,
     lon: yMap.get("lon") as number,
     name: yMap.get("name") as string | undefined,
+    isDayBreak: yMap.get("overnight") === true ? true : undefined,
   }));
 }