Upgrade to C++20 and fix all compilation warnings

- Upgrade from C++17 to C++20
- Remove unused helper functions (selectSpawnPreset, parseVec3Csv, parseYawPitchCsv)
- Mark unused parameters with [[maybe_unused]] attribute
- Remove unused variables (nameColor, currentRace, steppingUp, steppingDown, awayFromWallMotion)
- Fix all -Wunused-* warnings

Build now completes with zero warnings.

src/ui/character_create_screen.cpp

@@ -369,7 +369,6 @@ void CharacterCreateScreen::render(game::GameHandler& /*gameHandler*/) {
 
     // Appearance sliders
     updateAppearanceRanges();
-    game::Race currentRace = allRaces[raceIndex];
     game::Gender currentGender = static_cast<game::Gender>(genderIndex);
 
     ImGui::Text("Appearance");