[fix](trx-frontend-http): buffer decode messages until map-data plugins are ready

Eagerly load map-data plugins (AIS, APRS, VDES, HF-APRS) on startup and
buffer any decode history or live SSE messages that arrive before plugin
handlers register. Each plugin drains its pending buffer on init.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/plugins/hf-aprs.js

@@ -437,3 +437,4 @@ window.onServerHfAprs = function(pkt) {
 };
 
 renderHfAprsHistory();
+if (window._trxDrainPendingDecode) window._trxDrainPendingDecode("hf_aprs");