fix(awards): upper-case the DXCC main prefix (3D2/c → 3D2/C)

cty.dat spells some sub-entity suffixes in lower case; the awards prefix column
now shows them upper-cased like every other DXCC prefix.

internal/dxcc/manager.go

@@ -197,7 +197,9 @@ func (m *Manager) PrefixByDXCC() map[int]string {
 		}
 		if n := EntityDXCC(e.Name); n > 0 {
 			if _, dup := out[n]; !dup {
-				out[n] = p
+				// DXCC prefixes are conventionally upper-case; cty.dat spells some
+				// sub-entity suffixes in lower case (3D2/c → 3D2/C).
+				out[n] = strings.ToUpper(p)
 			}
 		}
 	}