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.cpp

@@ -0,0 +1,39 @@
+#include "brightness.h"
+#include "idle.h"
+#include <Preferences.h>
+#include <Arduino.h>
+
+// Four-step ramp. The default (index 2) is 200 — identical to the prior
+// hard-coded DISPLAY_DEFAULT_BRIGHTNESS, so cycling is purely additive.
+static const uint8_t LEVELS[] = {64, 128, 200, 255};
+#define LEVELS_COUNT (sizeof(LEVELS) / sizeof(LEVELS[0]))
+#define DEFAULT_IDX  2
+
+static uint8_t cur_idx = DEFAULT_IDX;
+
+void brightness_init(void) {
+    Preferences prefs;
+    prefs.begin("clawdmeter", true);
+    uint8_t saved_idx = prefs.getUChar("brt_idx", 0xFF);
+    prefs.end();
+
+    if (saved_idx < LEVELS_COUNT) cur_idx = saved_idx;
+    idle_set_awake_brightness(LEVELS[cur_idx]);
+    Serial.printf("Brightness init: level=%u (idx=%u)\n", LEVELS[cur_idx], cur_idx);
+}
+
+void brightness_cycle(void) {
+    cur_idx = (cur_idx + 1) % LEVELS_COUNT;
+
+    Preferences prefs;
+    prefs.begin("clawdmeter", false);
+    prefs.putUChar("brt_idx", cur_idx);
+    prefs.end();
+
+    idle_set_awake_brightness(LEVELS[cur_idx]);
+    Serial.printf("Brightness cycled: level=%u (idx=%u)\n", LEVELS[cur_idx], cur_idx);
+}
+
+uint8_t brightness_get(void) {
+    return LEVELS[cur_idx];
+}