feat(leaderboard): add custom public display name

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>

solitaire_data/src/settings.rs

@@ -231,6 +231,12 @@ pub struct Settings {
     /// cleanly to `None` via `#[serde(default)]`.
     #[serde(default)]
     pub last_difficulty: Option<DifficultyLevel>,
+    /// Custom public name displayed on the leaderboard. When `None`, the
+    /// player's server `username` is used instead. Trimmed to 32 characters
+    /// before submission. Older `settings.json` files written before this
+    /// field existed deserialize cleanly to `None` via `#[serde(default)]`.
+    #[serde(default)]
+    pub leaderboard_display_name: Option<String>,
 }
 
 fn default_draw_mode() -> DrawMode {
@@ -350,6 +356,7 @@ impl Default for Settings {
             disable_smart_default_size: false,
             replay_move_interval_secs: default_replay_move_interval_secs(),
             last_difficulty: None,
+            leaderboard_display_name: None,
         }
     }
 }