feat(power): RX duty-cycle watchdog, noise-floor recal, GPS duty-cycling

Three power-saving additions, prompted by comparing this fork's existing
RX duty-cycle support against IoTThinks/EasySkyMesh:

- RX duty-cycle watchdog: the SX126x's hardware RX<->sleep sequencer runs
  with no MCU polling, so a desync (a known failure mode) previously had
  nothing watching for it. A new watchdog samples the BUSY pin every tick;
  no transition for too long triggers a soft re-arm, then a full chip
  reset (with cached radio params reapplied, since std_init() resets to
  compiled firmware defaults) if that doesn't clear it. Soft/hard recovery
  counts surface on Tools > Diagnostics > Live as "RXPS wd s/h".

- Noise-floor recalibration during power-save: sampling was previously
  skipped entirely while duty-cycling, freezing int.thresh interference
  detection at whatever the floor was when power-save turned on. Now
  borrows a brief continuous-RX window once a minute to take a fresh
  reading before re-arming duty-cycle.

- GPS duty-cycling (Settings > System > "GPS pwr"): cycles GPS off between
  fixes instead of running it continuously. Each wake waits for a fix
  (capped at 60s) before sleeping again for the configured interval.
  Repurposes the long-dead NodePrefs::gps_interval byte rather than adding
  a new persisted field. A "is anything live using GPS right now" hold in
  UITask keeps GPS continuously on whenever trail recording, live-share,
  an armed Locator, or the Compass/Nearby-navigate view actually need a
  live fix, so none of those features degrade. Locator crossing-state is
  reset on each wake so a still-settling first fix can't read as a false
  geofence crossing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

examples/companion_radio/NodePrefs.h

@@ -62,7 +62,7 @@ struct NodePrefs {  // persisted to file
   uint8_t  buzzer_quiet;
   uint8_t  buzzer_volume;   // 0=min..4=max, default 4
   uint8_t  gps_enabled;      // GPS enabled flag (0=disabled, 1=enabled)
-  uint32_t gps_interval;     // GPS read interval in seconds
+  uint32_t gps_interval;     // GPS duty-cycle sleep window in seconds (0 = disabled, GPS stays continuous)
   uint8_t autoadd_config;    // bitmask for auto-add contacts config
   uint8_t rx_boosted_gain; // SX126x RX boosted gain mode (0=power saving, 1=boosted)
   uint8_t client_repeat;