feat: Save player name in localStorage — only ask once - #49

Summary

Persist the player name in localStorage after the first leaderboard submission so returning players see their name pre-filled on subsequent game-over screens.

Changes

• Added PLAYER_NAME_STORAGE_KEY (teeter_player_name) following the existing teeter_ naming convention
• Added loadPlayerName() and savePlayerName() helper functions mirroring the sensitivity settings pattern
• On score submission, the entered name is saved to localStorage
• On qualifying game-over, the name input is pre-filled with the saved name
• When pre-filled, the text is selected (nameInput.select()) for easy replacement
• First-time players still see an empty input field
• Applied to both js/main.js (localStorage leaderboard) and public/js/main.js (API leaderboard)

Acceptance Criteria

1. ✅ After submitting name, it is saved to localStorage
2. ✅ On subsequent qualifying game-overs, name input is pre-filled
3. ✅ Player can still edit the pre-filled name
4. ✅ First-time players see empty input
5. ✅ Input receives focus and text is selected when pre-filled
6. ✅ Key follows convention: teeter_player_name