Add keyboard backlight config and control

- Add user config for brightness and auto-on/off control.
- Auto-switch backlight on/off according to power state.

src/main.cpp

@@ -654,6 +654,9 @@ void setup() {
     powerMgr.setDimTimeout(userConfig.settings().screenDimTimeout);
     powerMgr.setOffTimeout(userConfig.settings().screenOffTimeout);
     powerMgr.setBrightness(userConfig.settings().brightness);
+    powerMgr.setKbBrightness(userConfig.settings().keyboardBrightness);
+    powerMgr.setKbAutoOn(userConfig.settings().keyboardAutoOn);
+    powerMgr.setKbAutoOff(userConfig.settings().keyboardAutoOff);
 
     // Step 24.5: GPS init
 #if HAS_GPS
@@ -903,6 +906,11 @@ void setup() {
         }
     }
 
+    if (userConfig.settings().keyboardAutoOn) {
+        // We are in ACTIVE power state here, switch keyboard backlight ON
+        keyboard.backlightOn();
+    }
+
     Serial.println("[BOOT] RatDeck ready");
     Serial.printf("[BOOT] Summary: radio=%s flash=%s sd=%s\n",
                   radioOnline ? "ONLINE" : "OFFLINE",