Fix compass heading direction, add magnetic declination and heading offset

- Fix atan2 sign bug: atan2(-yHeading, xHeading) per Freescale AN4248 eq.22.
  The previous atan2(yHeading, xHeading) produced counter-clockwise angles,
  causing East/West to be swapped relative to compass convention.

- Add imuSetMagneticDeclination() (T:146, field "decl") to correct the
  offset between magnetic north and true north (iPhone shows true north).
  Value is persisted in imuConfig.json across reboots.

- Add imuSetHeadingOffset() (T:147, field "off") to compensate for sensor
  mounting orientation on the rover. Also persisted.

- Persist decl and hOff in imuConfig.json (version 2, backwards compatible).

- Fix calibration feedback for web interface: T:126 now includes calDone=1
  (success) or calDone=0 (fail) once after calibration completes, plus
  decl and hOff fields. Calibration start message explains polling procedure.

- Remove duplicate #include <nvs_flash.h> and unused Adafruit ICM20948
  includes that caused build failures when libraries were not installed.

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

uart_ctrl.h

@@ -59,14 +59,20 @@ void jsonCmdReceiveHandler(){
 	case CMD_FEEDBACK_FLOW_INTERVAL:
 												setFeedbackFlowInterval(
 												jsonCmdReceive["cmd"]);break;
-	case CMD_UART_ECHO_MODE:
-												setCmdEcho(
-												jsonCmdReceive["cmd"]);break;
-	case CMD_CALI_MAG_START:
-												startMagCalibration();break;
-	case CMD_ARM_CTRL_UI: RoArmM2_uiCtrl(
-												jsonCmdReceive["E"],
-												jsonCmdReceive["Z"],
+	case CMD_UART_ECHO_MODE:
+												setCmdEcho(
+												jsonCmdReceive["cmd"]);break;
+	case CMD_CALI_MAG_START:
+												startMagCalibration();break;
+	case CMD_SET_MAG_DECLINATION:
+												setMagDeclination(
+												jsonCmdReceive["decl"]);break;
+	case CMD_SET_HEADING_OFFSET:
+												setHeadingOffset(
+												jsonCmdReceive["off"]);break;
+	case CMD_ARM_CTRL_UI: RoArmM2_uiCtrl(
+												jsonCmdReceive["E"],
+												jsonCmdReceive["Z"],
 												jsonCmdReceive["R"]
 												);break;
                         
@@ -515,4 +521,4 @@ void serialCtrl() {
       receivedData = "";
     }
   }
-}
+}