idle owns the panel brightness (it fades between a "full" level and 0), so
user brightness control routes through a new idle_set_awake_brightness():
idle now fades to a runtime awake level instead of the hard-coded
DISPLAY_DEFAULT_BRIGHTNESS, and the chosen level survives idle fades + wake.
brightness.{h,cpp} holds a 4-step ramp {64,128,200,255} (default 200 == the
old constant, so default behavior is unchanged) persisted to NVS, and applies
the level through idle. Wired to the PWR button in the next commit.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
