Add ble_hs_synced() guard to notifyAll()

Matches the guard already on notify() to prevent use-after-free
of _tx_char during a NimBLE host reset.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/ble_interface/platforms/NimBLEPlatform.cpp

@@ -1817,7 +1817,7 @@ bool NimBLEPlatform::notify(uint16_t conn_handle, const Bytes& data) {
 }
 
 bool NimBLEPlatform::notifyAll(const Bytes& data) {
-    if (!_tx_char) {
+    if (!ble_hs_synced() || !_tx_char) {
         return false;
     }