Document ground vehicle markers in README, refresh firmware.bin for v3.1.0

Adds a note to the "Hide ground vehicles" section about the new blue
square markers and matching legend entry, and refreshes the root
firmware.bin to match the v3.1.0 build output including the ground
vehicle marker feature.

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

README.md

@@ -189,6 +189,8 @@ Completely hides aircraft from specific airlines from the radar (they're also ex
 ### 🚗 Hide ground vehicles (ON/OFF)
 ADS-B data doesn't only contain aircraft – it can also include **airport ground vehicles** (follow-me cars, pushback tugs, etc., flagged by the API under a separate "C" category). This option hides them so the radar stays focused purely on air traffic.
 
+When shown, ground vehicles render as distinct **blue square markers** on the radar screen instead of the normal aircraft marker, with a matching entry in the legend - so they're never mistaken for a low-flying aircraft.
+
 ### 📶 WiFi Manager
 See the dedicated section above: [WiFi Manager](#-wifi-manager-up-to-3-saved-networks) – save up to 3 WiFi networks at once.