feat(power): CAD-windowed receive (battery saver) — phase 1

Add an optional low-power RX mode: instead of listening continuously, the
radio periodically runs a Channel Activity Detection scan and sits in standby
between scans; on detected activity it drops into a full receive to catch the
packet. Cuts average RX current several-fold (the dominant draw on this node).

- RadioLibWrapper: CAD state machine (scan → sleep window → rx) driven from
  loop(), which runs before recvRaw each cycle. `state` stays STATE_RX through
  all phases so the dispatcher's not-in-RX watchdog never trips. Re-arm routes
  through armRecv() in both startRecv() and recvRaw(); toggling on/off switches
  mode safely at the next idle. Falls back to continuous RX if CAD is
  unsupported. setPowerSaving()/getPowerSaving() control it.
- Software CSMA is already disabled on this firmware (interference threshold 0),
  so duty-cycling doesn't affect listen-before-talk; airtime budget still gates
  TX, and isReceivingPacket() still defers TX while a packet is mid-air.
- Companion: new rx_powersave pref (schema 0xC0DE0008), Settings › Radio ›
  "Pwr save" toggle, applied at boot (MyMesh) and on change (UITask).

CAD window = 45 ms (< the 16-symbol preamble at SF8/BW62.5) and CAD params use
RadioLib defaults — both will want on-hardware tuning + a before/after current
measurement to confirm reception is intact.

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

examples/companion_radio/ui-new/UITask.cpp

@@ -2066,6 +2066,11 @@ void UITask::applyTxPower() {
   radio_driver.setTxPower(_node_prefs->tx_power_dbm);
 }
 
+void UITask::applyPowerSave() {
+  if (_node_prefs == NULL) return;
+  radio_driver.setPowerSaving(_node_prefs->rx_powersave);
+}
+
 void UITask::applyBrightness() {
   if (_display != NULL && _node_prefs != NULL) {
     _display->setBrightness(_node_prefs->display_brightness);