feat(engine): card-art thumbnails in the theme picker

Settings → Cosmetic's theme picker showed only the theme name. Now
each chip carries a small Ace-of-Spades + back preview pair so the
player can see what each theme looks like before switching.

A new ThemeThumbnailCache resource keys per-theme by id and stores
two Handle<Image>s (ace + back) rasterised at thumbnail resolution
via the existing rasterize_svg path. Generation runs once per
theme registration in theme_plugin; subsequent picker re-spawns
just look up the cached handles. Themes that lack one of the
preview SVGs (broken user theme) get a Handle::default() placeholder
rather than crashing — the placeholder renders as a transparent
rectangle the same size as the missing thumbnail.

The picker chip spawn loop in settings_plugin reads the cache and
renders the pair as two child sprites above the chip text. The
selected-theme chip's existing STATE_SUCCESS tint sits behind the
thumbnails; contrast stays readable.

Asset-source plumbing in assets/sources.rs and assets/mod.rs picks
up the new bytes-loading helper that the thumbnail generator uses
for embedded:// theme assets at startup time (before AssetServer is
fully initialised).

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

solitaire_engine/src/assets/mod.rs

@@ -11,8 +11,8 @@ pub mod svg_loader;
 pub mod user_dir;
 
 pub use sources::{
-    populate_embedded_default_theme, register_theme_asset_sources, AssetSourcesPlugin,
-    DEFAULT_THEME_MANIFEST_URL, USER_THEMES,
+    default_theme_svg_bytes, populate_embedded_default_theme, register_theme_asset_sources,
+    AssetSourcesPlugin, DEFAULT_THEME_MANIFEST_URL, USER_THEMES,
 };
 pub use svg_loader::{rasterize_svg, SvgLoader, SvgLoaderError, SvgLoaderSettings};
 pub use user_dir::{set_user_theme_dir, user_theme_dir};