feat(engine): empty-state copy + onboarding hints across panels

- Leaderboard empty state: replace single muted line with a two-tier
  "Be the first on the leaderboard." headline + body invite.
- Achievements panel: surface a first-launch hint above the grid until
  the player unlocks anything, so the greyed-out rows aren't context-free.
- Volume hotkeys ([/]): emit an InfoToastEvent with the new percentage so
  off-panel adjustments give visible feedback (previously silent).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/leaderboard_plugin.rs

@@ -501,7 +501,12 @@ fn spawn_leaderboard_screen(
                 }
                 LeaderboardResource::Loaded(rows) if rows.is_empty() => {
                     body.spawn((
-                        Text::new("No entries yet \u{2014} sync and opt in to appear here."),
+                        Text::new("Be the first on the leaderboard."),
+                        font_status.clone(),
+                        TextColor(TEXT_PRIMARY),
+                    ));
+                    body.spawn((
+                        Text::new("Win a game and opt in to appear here."),
                         font_row.clone(),
                         TextColor(TEXT_SECONDARY),
                     ));