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/platformio.ini

@@ -21,12 +21,6 @@ build_flags =
     -DLV_CONF_SKIP
     -DLV_COLOR_DEPTH=16
     -DLV_USE_LOG=0
-    -DLV_FONT_MONTSERRAT_12=1
-    -DLV_FONT_MONTSERRAT_14=1
-    -DLV_FONT_MONTSERRAT_16=1
-    -DLV_FONT_MONTSERRAT_20=1
-    -DLV_FONT_MONTSERRAT_24=1
-    -DLV_FONT_MONTSERRAT_28=1
     -DLV_USE_BAR=1
     -DLV_USE_LABEL=1
     -DLV_USE_ARC=1