brightness: user-cyclable level persisted to NVS, via idle

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>

firmware/src/brightness.h

@@ -0,0 +1,10 @@
+#pragma once
+#include <stdint.h>
+
+// User-controlled display brightness, persisted to NVS. The middle (PWR)
+// button short-press cycles through the levels via brightness_cycle().
+// idle owns the actual panel brightness, so this routes the chosen level
+// through idle_set_awake_brightness().
+void    brightness_init(void);    // load saved level from NVS and apply
+void    brightness_cycle(void);   // advance to next level, save, apply
+uint8_t brightness_get(void);     // current PWM level (0..255)