R1350 YA105 LED control fix — adds Xiaomi LED board mapping.

Changes from LED-CORRECTED-20260720:
  - Added xiaomi,mi-router-4-pro-r1350 case in board.d/01_leds:
      yellow:system (GPIO2) → WAN link indicator
      yellow:function (GPIO4) → 2.4GHz WiFi activity
      blue:function (GPIO5) → 5GHz WiFi activity
      blue:system (GPIO3) → system running (handled by preinit alias, always on when healthy)

GPIO mapping (all active-low, unchanged from previous build):
  GPIO2 = yellow:system   GPIO3 = blue:system
  GPIO4 = yellow:function GPIO5 = blue:function

