Fix ADSB altitude integer overflow and add on-ground indicator (#3275)

* Initial plan

* Fix ADSB altitude integer overflow and add on-ground indicator

- Fix display overflow: use to_string_dec_int (signed) instead of
  to_string_dec_uint (unsigned) for altitude display - per ICAO Annex 10
  Vol IV 3.1.2.6.5.4, altitude = 25N-1000 can be as low as -1000ft
- Remove incorrect altitude clamp (altitude < 0 => 0) in DF 0/4/20 decoder
- Add on_ground flag to AircraftRecentEntry; set from TC 5-8 (surface
  position) messages, cleared on TC 9-22 (airborne position) messages
- Display 'GND' in altitude column when aircraft is on ground
- Clear altitude to 0 when on_ground is set to prevent stale data in map
  color calculation

Closes #3274

* Revert on-ground indicator; keep only the overflow and clamp fixes

Remove on_ground field, TC 5-8 handling, and "GND" display per review
feedback. Only keep the two minimal bug fixes:
- Signed altitude display (to_string_dec_int vs to_string_dec_uint)
- Remove erroneous altitude < 0 clamp; update comment to show the math

firmware/application/apps/ui_adsb_rx.cpp

@@ -75,7 +75,7 @@ void RecentEntriesTable<AircraftRecentEntries>::draw(
     uint8_t firstcolwidth = columns.at(0).second;
     ipc.resize(firstcolwidth, ' ');  // Make sure this is always match the first column's width that is dynamic.
 
-    entry_string += ipc + to_string_dec_uint((unsigned int)(entry.pos.altitude / 100), 4);
+    entry_string += ipc + to_string_dec_int((int32_t)(entry.pos.altitude / 100), 4);
 
     if (entry.velo.type == SPD_IAS && entry.pos.alt_valid) {  // IAS can be converted to TAS
         // It is generally accepted that for every thousand feet of altitude,
@@ -668,9 +668,8 @@ void ADSBRxView::on_frame(const ADSBFrameMessage* message) {
                         (raw_data[3] & 15);
 
                 // The final altitude is due to the resulting number multiplied by 25, minus 1000.
+                // altitude = 25N - 1000 (ft); minimum is -1000 ft when N=0 (Q=1, M=0 per ICAO Annex 10).
                 altitude = 25 * n - 1000;
-                if (altitude < 0)
-                    altitude = 0;
             }  // else N is an 11 bit Gillham coded altitude
         }