fix(engine): restore Dark as default theme; migrate stale theme IDs

- default_theme_id() returns "dark" (was briefly "classic" after the
  rename commit 20b7a61)
- sanitized() migrates "default" and "classic" → "dark" so existing
  settings.json files are upgraded automatically on next launch
- Registry lists Dark first so the Settings picker opens with it at top
- Classic remains available as an option in the picker

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_data/src/settings.rs

@@ -143,10 +143,10 @@ pub struct Settings {
     #[serde(default)]
     pub window_geometry: Option<WindowGeometry>,
     /// Identifier of the active card-art theme. Matches `meta.id` from
-    /// the theme's `theme.ron` manifest. `"classic"` and `"dark"` are
+    /// the theme's `theme.ron` manifest. `"dark"` and `"classic"` are
     /// always present; user-supplied themes register under their own ids.
-    /// Older `settings.json` files that stored `"default"` will fall
-    /// back to the dark embedded theme at runtime.
+    /// Older `settings.json` files that stored `"default"` or `"classic"`
+    /// are migrated to `"dark"` by [`Settings::sanitized`].
     #[serde(default = "default_theme_id")]
     pub selected_theme_id: String,
     /// Set to `true` once the achievement-onboarding info-toast has been
@@ -272,7 +272,7 @@ fn default_music_volume() -> f32 {
 }
 
 fn default_theme_id() -> String {
-    "classic".to_string()
+    "dark".to_string()
 }
 
 /// Default tooltip-hover dwell delay in seconds. Mirrors
@@ -395,6 +395,13 @@ impl Settings {
     /// their respective ranges after deserialization or hand-editing of
     /// `settings.json`.
     pub fn sanitized(self) -> Self {
+        // Migrate stale theme IDs: "default" was removed when the theme was
+        // renamed to "dark"; "classic" was briefly the default before "dark"
+        // was restored as the shipped default.
+        let selected_theme_id = match self.selected_theme_id.as_str() {
+            "default" | "classic" => "dark".to_string(),
+            _ => self.selected_theme_id,
+        };
         Self {
             sfx_volume: self.sfx_volume.clamp(0.0, 1.0),
             music_volume: self.music_volume.clamp(0.0, 1.0),
@@ -407,6 +414,7 @@ impl Settings {
             replay_move_interval_secs: self
                 .replay_move_interval_secs
                 .clamp(REPLAY_MOVE_INTERVAL_MIN_SECS, REPLAY_MOVE_INTERVAL_MAX_SECS),
+            selected_theme_id,
             ..self
         }
     }