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>

json_cmd.h

@@ -112,12 +112,20 @@
 // set the echo mode of recving new cmd.
 // 0: [default]off
 // 1: on
-// {"T":143,"cmd":0}
-#define CMD_UART_ECHO_MODE	143
-
-// start magnetometer calibration and save to flash after rotation
-// {"T":145}
-#define CMD_CALI_MAG_START	145
+// {"T":143,"cmd":0}
+#define CMD_UART_ECHO_MODE	143
+
+// start magnetometer calibration and save to flash after rotation
+// {"T":145}
+#define CMD_CALI_MAG_START	145
+
+// set magnetic declination in degrees (find at magnetic-declination.com)
+// {"T":146,"decl":4.5}
+#define CMD_SET_MAG_DECLINATION	146
+
+// set fixed heading offset in degrees (compensates sensor mounting angle)
+// {"T":147,"off":90.0}
+#define CMD_SET_HEADING_OFFSET	147
 
 
 
@@ -576,4 +584,4 @@
 // === === === mainType & moduleType settings. === === ===
 // {"T":900,"main":1,"module":0}
 // main_type: 1-WAVE ROVER, 2-UGV02, 3-UGV01
-#define CMD_MM_TYPE_SET 900
+#define CMD_MM_TYPE_SET 900