v2 firmware: HA command channel, battery screen, dynamic Home buttons, tilt-dimmer

Watch-side features developed on the AMOLED-2.06 and shared across all boards:

- BLE command channel (…0005, notify watch→PC) via ble_send_command(); the
  watch stays "dumb" and the daemon maps commands to Home Assistant.
- Battery detail screen + protective low-voltage cutoff; power HAL gains
  battery_mv() / shutdown(), with battery_est.{h,cpp} for the time-left anchor.
- Home screen: dynamic 2-column button grid driven by the desktop config
  (RX "btns" labels); tap notifies {"cmd":"btn","i":N}.
- Dimmer screen: IMU tilt-joystick over the light's brightness / color temp.
  imu_hal_read_accel() added to the HAL (real on 2.06/2.16, no-op elsewhere);
  streams absolute {"cmd":"bri"|"ct","v":..}; daemon seeds the dial via "dim".
  Per-board tilt axis is calibrated with the DIM_CALIB overlay (off by default).

Multi-board fix: enable LV_FONT_MONTSERRAT_20/28 on the 2.16, 1.8 and C6 envs.
Shared ui.cpp uses these glyphs for the launcher tiles and back chevron, so
those three boards had failed to compile since the Phase-2 UI landed. All four
envs now build clean. README: correct the Windows pairing name to "Clawdmeter".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

firmware/src/boards/waveshare_amoled_216/imu.cpp

@@ -64,3 +64,8 @@ void imu_hal_tick(void) {
 }
 
 uint8_t imu_hal_rotation_quadrant(void) { return current_rotation; }
+
+bool imu_hal_read_accel(float* x, float* y, float* z) {
+    if (!imu_ok || !imu.getDataReady()) return false;
+    return imu.getAccelerometer(*x, *y, *z);
+}