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/achievement_plugin.rs

@@ -474,9 +474,29 @@ fn spawn_achievements_screen(
         ..default()
     };
 
+    let any_unlocked = records.iter().any(|r| r.unlocked);
+
     let scrim = spawn_modal(commands, AchievementsScreen, Z_MODAL_PANEL, |card| {
         spawn_modal_header(card, header, font_res);
 
+        // First-time hint — shown until the player has unlocked anything.
+        // The list itself describes individual rewards, but a top-level
+        // explanation gives newer players context for the otherwise dense
+        // greyed-out grid.
+        if !any_unlocked {
+            card.spawn((
+                Text::new(
+                    "Complete games and try new modes to unlock achievements and rewards.",
+                ),
+                TextFont {
+                    font: font_res.map(|f| f.0.clone()).unwrap_or_default(),
+                    font_size: TYPE_CAPTION,
+                    ..default()
+                },
+                TextColor(TEXT_SECONDARY),
+            ));
+        }
+
         // Scrollable body — the achievements list grows to ~19 rows which
         // overflows the modal on the 800x600 minimum window. Wrapping the
         // row list in an `Overflow::scroll_y()` Node with a constrained