feat(engine): unify dismiss verb to Done and warm onboarding CTA to Let's play

The Help modal previously used "Close" while the other five overlay
modals (Home, Stats, Achievements, Settings, Profile, Leaderboard)
used "Done"; standardising on "Done" removes the outlier.

The final onboarding slide changes from "Start playing" to
"Let's play". The microcopy audit suggested matching the win modal's
"Play Again", but that verb is semantically wrong on first launch —
the player has not yet played. "Let's play" reads warmer and matches
the project's Balatro-tone direction without overloading "Play Again"
across two contexts that mean different things.

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

solitaire_engine/src/onboarding_plugin.rs

@@ -166,7 +166,7 @@ fn handle_onboarding_buttons(
     }
 
     if skip_pressed || (next_pressed && slide_index.0 == SLIDE_COUNT - 1) {
-        // Skip or final-slide "Start playing" — complete onboarding.
+        // Skip or final-slide "Let's play" — complete onboarding.
         complete_onboarding(
             &mut commands,
             &screens,
@@ -412,7 +412,7 @@ fn spawn_slide_hotkeys(commands: &mut Commands, font_res: Option<&FontResource>)
             spawn_modal_button(
                 actions,
                 OnboardingNextButton,
-                "Start playing",
+                "Let's play",
                 Some("→"),
                 ButtonVariant::Primary,
                 font_res,