Add timesync burst handling and sender-only timeout

- Add TimeSync fault code and labels in UI/SD/web docs
- Trigger receiver beacon bursts on sender drift, but keep errors sender-local
- Sender flags TimeSync only after TIME_SYNC_ERROR_TIMEOUT_MS

src/web_server.cpp

@@ -613,7 +613,7 @@ static void handle_manual() {
   html += "<li>Battery: percent with voltage in V.</li>";
   html += "<li>RSSI/SNR: LoRa link quality from last packet.</li>";
   html += "<li>err_tx: sender-side LoRa TX error counter.</li>";
-  html += "<li>err_last: last error code (0=None, 1=MeterRead, 2=Decode, 3=LoraTx).</li>";
+  html += "<li>err_last: last error code (0=None, 1=MeterRead, 2=Decode, 3=LoraTx, 4=TimeSync).</li>";
   html += "<li>faults m/d/tx: receiver-side counters (meter read fails, decode fails, LoRa TX fails).</li>";
   html += "<li>faults last: last receiver-side error code (same mapping as err_last).</li>";
   html += "</ul>";