Add Power::forceScreenOff() for manual screen-off triggers

New public method that transitions to SCREEN_OFF on demand. A
_justWokeFromOff flag is set in activity() and cleared at the end
of loop() so a single keypress that woke the screen can't immediately
re-blank it via a hotkey/long-press handler running in the same tick.

Pure infrastructure — no UI binding included; consumers can wire it
to whatever input event they want (long-press, hotkey, etc.).

src/hal/Power.cpp

@@ -48,11 +48,22 @@ uint8_t Power::percentToPWM(uint8_t pct) const {
 
 void Power::activity() {
     _lastActivity = millis();
+    if (_state == SCREEN_OFF) {
+        _justWokeFromOff = true;
+    }
     if (_state != ACTIVE) {
         setState(ACTIVE);
     }
 }
 
+void Power::forceScreenOff() {
+    if (_justWokeFromOff) {
+        _justWokeFromOff = false;
+        return;
+    }
+    setState(SCREEN_OFF);
+}
+
 void Power::weakActivity() {
     _lastActivity = millis();
     // Trackball wakes from DIM but not from SCREEN_OFF
@@ -96,6 +107,8 @@ void Power::loop() {
         case SCREEN_OFF:
             break;
     }
+
+    _justWokeFromOff = false;
 }
 
 void Power::setState(State newState) {

src/hal/Power.h

@@ -15,6 +15,9 @@ public:
     void activity();
     // Weak activity = trackball — wakes from DIM only, not SCREEN_OFF
     void weakActivity();
+    // Manual screen-off; no-op if activity() just woke from SCREEN_OFF this tick
+    // so a single keypress can't both wake and re-sleep.
+    void forceScreenOff();
 
     // Battery
     float batteryVoltage() const;
@@ -57,4 +60,5 @@ private:
     static constexpr uint8_t DIM_PWM = 40;  // ~15% PWM when dimmed
     bool _kbAutoOn = false;
     bool _kbAutoOff = false;
+    bool _justWokeFromOff = false;
 };