Remove dead functions and unused Montserrat font flags

Six public API functions had no callers anywhere in the firmware:
splash_prev, splash_set_active, splash_current_name, splash_count,
ble_is_connected, imu_set_rotation, power_is_usb_connected,
power_usb_changed. Trimming them lets power.cpp drop the VBUS state
machine (cached_usb, usb_changed_flag, last_vbus_ms,
enableVbusVoltageMeasure). usage_rate_reset becomes file-static since
the only caller is in usage_rate.cpp itself. Montserrat font flags in
platformio.ini were enabled but never referenced.

firmware/src/usage_rate.h

@@ -10,6 +10,3 @@ void usage_rate_sample(float session_pct);
 // 0 = idle, 1 = normal, 2 = active, 3 = heavy.
 // Defaults to 0 when the buffer doesn't have enough samples yet.
 int usage_rate_group(void);
-
-// Clear the buffer (e.g. when the 5-hour session window resets).
-void usage_rate_reset(void);