chore(platformio): exclude MQTT-specific helper files from build

platformio.ini

@@ -49,6 +49,9 @@ build_flags = -w -DNDEBUG -DRADIOLIB_STATIC_ONLY=1 -DRADIOLIB_GODMODE=1
 build_src_filter =
   +<*.cpp>
   +<helpers/*.cpp>
+  ; MQTT-only sources (need Timezone/NTPClient); observer envs add them back.
+  -<helpers/MQTTMessageBuilder.cpp>
+  -<helpers/JWTHelper.cpp>
   +<helpers/radiolib/*.cpp>
   +<helpers/bridges/BridgeBase.cpp>
   +<helpers/ui/MomentaryButton.cpp>