Fix screen wake on idle, add regional frequency presets and Hz input

Screen idle:
- Remove powerMgr.activity() from radio yield callback — screen no
  longer wakes on LoRa RX, auto-announce, or RSSI polls
- Add [POWER] state transition logging for on-device diagnostics

Radio regions:
- Add 4-region system: Americas (915), Europe (868), Australia (915),
  Asia (923) with timezone-to-region mapping
- Region picker in Radio settings (always visible)
- Presets now apply region-appropriate frequency + modulation params
- Timezone change warns if it suggests a different radio region

Frequency input (developer mode):
- Hz-precision input (9 digits) displayed as MHz with trimmed decimals
- Left/right arrows step by 125 kHz, digit keys for direct entry
- Inspired by @strijar in https://github.com/ratspeak/ratdeck/pull/12

src/hal/Power.cpp

@@ -91,6 +91,8 @@ void Power::loop() {
 
 void Power::setState(State newState) {
     if (newState == _state) return;
+    const char* names[] = {"ACTIVE", "DIMMED", "SCREEN_OFF"};
+    Serial.printf("[POWER] %s -> %s\n", names[_state], names[newState]);
     State oldState = _state;
     _state = newState;