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>

IMU.h

@@ -20,23 +20,28 @@ typedef struct imu_st_sensor_data_float
   float Z;
 }IMU_ST_SENSOR_DATA_FLOAT;
 
-void imuInit();
-void imuDataGet(EulerAngles *pstAngles, 
-                IMU_ST_SENSOR_DATA_FLOAT *pstGyroRawData,
-                IMU_ST_SENSOR_DATA_FLOAT *pstAccelRawData,
-                IMU_ST_SENSOR_DATA *pstMagnRawData); 
-bool imuRecalibrate();
-void imuGetMagnOffsets(IMU_ST_SENSOR_DATA *pstMagnOffset);
-void imuSetMagnOffsets(int16_t offsetX, int16_t offsetY, int16_t offsetZ);
-bool imuHasHeadingCalibration();
-bool imuHasStoredMagnCalibration();
-bool imuIsMagnCalibrationRunning();
-uint8_t imuGetMagnCalibrationProgress();
-bool imuStartMagnCalibration(uint32_t durationMs = 12000);
-bool imuLoadMagnCalibration();
-bool imuSaveMagnCalibration();
-float imuGetTemperature();
-void imuAHRSupdate(float gx, float gy, float gz, float ax, float ay, float az, float mx, float my, float mz);
-float invSqrt(float x);
-
-#endif
+void imuInit();
+void imuDataGet(EulerAngles *pstAngles, 
+                IMU_ST_SENSOR_DATA_FLOAT *pstGyroRawData,
+                IMU_ST_SENSOR_DATA_FLOAT *pstAccelRawData,
+                IMU_ST_SENSOR_DATA *pstMagnRawData); 
+bool imuRecalibrate();
+void imuGetMagnOffsets(IMU_ST_SENSOR_DATA *pstMagnOffset);
+void imuSetMagnOffsets(int16_t offsetX, int16_t offsetY, int16_t offsetZ);
+bool imuHasHeadingCalibration();
+bool imuHasStoredMagnCalibration();
+bool imuIsMagnCalibrationRunning();
+uint8_t imuGetMagnCalibrationProgress();
+bool imuStartMagnCalibration(uint32_t durationMs = 12000);
+bool imuLoadMagnCalibration();
+bool imuSaveMagnCalibration();
+float imuGetTemperature();
+void imuAHRSupdate(float gx, float gy, float gz, float ax, float ay, float az, float mx, float my, float mz);
+float invSqrt(float x);
+void imuSetMagneticDeclination(float deg);
+float imuGetMagneticDeclination();
+void imuSetHeadingOffset(float deg);
+float imuGetHeadingOffset();
+int8_t imuPopLastCalStatus();
+
+#endif