feat(power): battery saving — hardware duty-cycle RX + adaptive TX power

Two independent, default-off toggles under Settings › Radio.

Pwr save: hardware RX duty-cycle (SX126x SetRxDutyCycle via
startReceiveDutyCycleAuto). The chip cycles RX↔sleep and wakes on a preamble —
no MCU state machine; recvRaw reads the packet exactly as in continuous RX.
Falls back to continuous RX on non-SX126x. (Replaces an earlier software-CAD
state machine that fought the hardware: polling a warm-sleeping chip gave a
phantom-busy channel that stalled TX ~4 s and dropped ACKs in the scan gaps.)

Auto pwr: Adaptive Power Control. tx_power_dbm becomes a ceiling; actual TX
power tracks the reverse-link SNR margin (measured above the per-SF demod floor,
EWMA-smoothed, proportional step with a deadband). Feedback comes from direct /
room-server ACKs and, for channels (no ACK), from hearing a repeater rebroadcast
our own flood; a lost confirmation ramps power back up so channel sends can't get
stranded below what the repeaters can hear.

Prefs schema 0xC0DE0009 (rx_powersave, tx_apc). Radio page / name bar show the
live TX power; noise floor reads n/a while duty-cycling.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.h

@@ -249,6 +249,8 @@ public:
   void setBuzzerVolumeLevel(uint8_t level);
   uint8_t getBuzzerVolume() const { return _node_prefs ? _node_prefs->buzzer_volume : 4; }
   void applyTxPower();
+  void applyPowerSave();   // hardware duty-cycle RX on/off from prefs
+  void applyApc();         // Adaptive Power Control on/off from prefs
   void applyFont();
   void applyRotation();
   void applyFullRefreshInterval();