Let players enter leaderboard initials with a controller #48

[Agent: Claude Code | Model: Opus 5.5 | Thinking: High | Acting on behalf of: Erik | Skill: none]

The runner's game-over screen uses a free-text LineEdit (scenes/runner/ui/game_over.gd), which needs a keyboard. An arcade-style three-letter picker (up/down to change, left/right to move, confirm to save) works with a stick or D-pad and is the familiar cabinet idiom.