fix(engine): use classic theme fallback in load_initial_theme

SettingsResource is not yet available at Startup, so load_initial_theme
fell back to "dark" on every run. On AMOLED the dark back (▒151515) is
invisible, showing only a 24×32 px red badge — the "tiny red squares"
bug. Cascade-collapse and top-row legibility were visual consequences of
the same invisible face-down cards, not layout bugs.

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

solitaire_engine/src/theme/plugin.rs

@@ -129,7 +129,7 @@ fn load_initial_theme(
     let id = settings
         .as_deref()
         .map(|s| s.0.selected_theme_id.as_str())
-        .unwrap_or("dark");
+        .unwrap_or("classic");
     let url = bundled_theme_url(id)
         .map(str::to_string)
         .unwrap_or_else(|| format!("themes://{id}/theme.ron"));