Adds `leaderboard_display_name: Option<String>` to `Settings` (serde
default = None, backwards-compatible). When set, this name is submitted
to the server on opt-in instead of the player's username, giving players
a separate public identity on the leaderboard.
Engine changes:
- `handle_opt_in_button` prefers `leaderboard_display_name` over username
- Leaderboard panel shows "Public name: X" row with "Set Name" button
- "Set Name" opens a modal with a single text-input field (32-char max)
- Save/Cancel buttons write to SettingsResource and persist to disk
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
funman300
03be4fcc67