fix: RX silence 15s warning fired ~30 times per window

The 10s silence warning used a one-shot bool guard, but the 15s warning
used a 500ms time window — firing every frame (~30 times at 60fps).
Added rxSilence15sLogged_ guard consistent with the 10s pattern.

include/game/game_handler.hpp

@@ -3047,6 +3047,7 @@ private:
     // ---- RX silence detection ----
     std::chrono::steady_clock::time_point lastRxTime_{};
     bool rxSilenceLogged_ = false;
+    bool rxSilence15sLogged_ = false;
 
     // ---- XP tracking ----
     uint32_t playerXp_ = 0;