idle: add IDLE_SLEEP_WHEN_CHARGING and IDLE_WAKE_ON_TOUCH

Addresses review feedback on #24:

- New config IDLE_SLEEP_WHEN_CHARGING (default false): while USB is
  plugged in we don't enter sleep and we wake immediately if power
  comes back. Covers both "I like watching Clawd animations on my
  plugged-in desk device" and "alternative hardware without a battery
  is always on USB, sleep would be odd."

- New config IDLE_WAKE_ON_TOUCH (default true): a touch on the dark
  panel wakes the device. The first touch is swallowed (mirrors the
  button wake-consumption), so tapping a dark screen wakes without
  also toggling splash<->usage. Set false to keep the original
  pets/sleeves-safe behaviour where touch is fully ignored while
  asleep.

power.{h,cpp} grow a power_is_vbus_in() helper backed by
pmu.isVbusIn() — distinct from isCharging() so the no-battery case
("plugged in, nothing to charge") is detected correctly.

firmware/src/idle.cpp

@@ -2,6 +2,7 @@
 #include "idle.h"
 #include "idle_cfg.h"
 #include "display_cfg.h"   // declares `extern Arduino_CO5300 *gfx;`
+#include "power.h"
 
 enum IdleState {
     STATE_AWAKE,
@@ -69,6 +70,16 @@ bool idle_is_asleep(void) {
 void idle_tick(void) {
     uint32_t now = millis();
 
+    // While on USB power (if configured), don't sleep — and wake from sleep
+    // when power comes back. Treats USB-in as continuous activity.
+    if (!IDLE_SLEEP_WHEN_CHARGING && power_is_vbus_in()) {
+        last_activity_ms = now;
+        if (state == STATE_ASLEEP || state == STATE_FADING_OUT) {
+            begin_fade(DISPLAY_DEFAULT_BRIGHTNESS, now);
+            state = STATE_FADING_IN;
+        }
+    }
+
     switch (state) {
     case STATE_AWAKE:
         if (now - last_activity_ms >= IDLE_TIMEOUT_MS) {