Add distinct marker and legend entry for ground vehicles on radar

Ground vehicles (ADS-B emitter category "C*") now render as filled
blue squares instead of the normal circle+arrowhead aircraft marker,
using a fixed color independent of altitude (they're practically
always at 0ft, which would otherwise collide with the low-altitude
green). A second legend line explains the marker, but only when
ground vehicles are actually visible (Flight Options > "Hide ground
vehicles" off) - the info bar height, and therefore the radar circle
size, is otherwise unchanged from before.

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

src/i18n_en.h

@@ -230,6 +230,8 @@ static const char* const I18N_EN[] = {
     ". From the next boot, WiFi will connect automatically.",
 
     "e.g. Home, Work, Holiday home",
+
+    "Ground vehicle",
 };
 
 static_assert(sizeof(I18N_EN) / sizeof(I18N_EN[0]) == (size_t)StringId::COUNT,