fix(room_server): support RX boosted gain

The room server never supported RX boosted gain, while the repeater
does. Three consequences on SX1262/SX1268 boards:

- boosted gain was never applied to the radio at boot, so a room server
  ran in power-saving RX mode while an identical repeater ran boosted -
  a real receive-sensitivity difference with nothing pointing at it
- _prefs.rx_boosted_gain was never initialised (the prefs are memset to
  0 before defaults are set), so it defaulted to off
- 'set radio.rxgain on' replied "Error: unsupported" but CommonCLI had
  already written the value to prefs and saved them, so a stale setting
  survived reboot and was never applied

Mirror the repeater's implementation: initialise the pref default under
the same USE_SX1262/USE_SX1268 and SX126X_RX_BOOSTED_GAIN guards, apply
it during radio init, and override setRxBoostedGain() so the CLI reports
success and takes effect.

Built Heltec_v3_room_server (SX1262) and LilyGo_T3S3_sx1276_room_server
(guards compile out cleanly).

examples/simple_room_server/MyMesh.h

@@ -206,6 +206,7 @@ public:
 
   void dumpLogFile() override;
   void setTxPower(int8_t power_dbm) override;
+  bool setRxBoostedGain(bool enable) override;
 
   void formatNeighborsReply(char *reply) override {
     strcpy(reply, "not supported");