fix(engine): unify light/dark theme manifest URL resolution

- set_theme resolved every id to themes:// — the bundled dark/classic
  themes live at embedded://, so switching to them via the public API
  would NotFound and silently keep the old theme. All three load paths
  now share one theme_manifest_url resolver.
- load_initial_theme's settings-absent fallback was "classic", a
  leftover from v0.33 when classic was the default; it now derives from
  Settings::default() so it can't drift from the data crate again, with
  a test pinning the default id to a bundled theme.
- settings.rs doc no longer claims "classic" is migrated to "dark";
  only the pre-rename "default" id is rewritten.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_data/src/settings.rs

@@ -161,8 +161,10 @@ pub struct Settings {
     /// Identifier of the active card-art theme. Matches `meta.id` from
     /// 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"` or `"classic"`
-    /// are migrated to `"dark"` by [`Settings::sanitized`].
+    /// Older `settings.json` files that stored `"default"` (the
+    /// pre-rename id of the dark theme) are migrated to `"dark"` by
+    /// [`Settings::sanitized`]; `"classic"` is a valid player choice
+    /// and is never rewritten.
     #[serde(default = "default_theme_id")]
     pub selected_theme_id: String,
     /// Set to `true` once the achievement-onboarding info-toast has been