feat(engine): ThemePlugin + ActiveTheme integration (Card theme phase 4)

Implements Phase 4 of CARD_PLAN.md — the runtime hook that loads the
default theme on startup and refreshes the card-rendering pipeline
whenever the active theme changes.

solitaire_engine/src/theme/plugin.rs
  ThemePlugin
    init_asset::<CardTheme>, register_asset_loader for SvgLoader and
    CardThemeLoader, Startup load_default_theme, and Update
    sync_card_image_set_with_active_theme.
  ActiveTheme(Handle<CardTheme>)
    Resource pointing at the currently-loaded theme.
  set_theme(commands, asset_server, theme_id)
    Public API for switching themes — formats the URL as
    `themes://<theme_id>/theme.ron` and updates the resource.

Integration approach: rather than refactor every `card_plugin.rs`
spawn site to read from `Assets<CardTheme>` directly, the sync system
writes the theme's face/back image handles into the existing
`CardImageSet` resource on `AssetEvent::LoadedWithDependencies` /
`Modified`, then fires `StateChangedEvent`. The existing
`sync_cards_on_change` pipeline rebuilds card sprites from the new
handles on the next tick — observable behaviour matches the plan's
intent (theme switches propagate immediately) while keeping
card_plugin's 1929-line surface area untouched.

Theme.back is mapped onto `CardImageSet.backs[0]` (the default-back
slot xCards previously occupied); `backs[1..=4]` are the
asset-generator patterns and remain user-selectable independent of
the active theme.

Added to solitaire_app/main.rs as `add_plugins(ThemePlugin)` after
`AssetSourcesPlugin` so the asset sources are registered before the
default-theme load is dispatched.

6 new tests covering suit/rank index mapping (matching the
`card_plugin` doc-commented `[suit][rank]` layout), empty-theme
no-panic, back-slot overwrite, and the URL format from `set_theme`.

cargo build / clippy --workspace --all-targets -- -D warnings / test
--workspace all green (950 passed, 0 failed, 9 ignored).

solitaire_engine/src/lib.rs

@@ -44,6 +44,7 @@ pub use assets::{
     populate_embedded_default_theme, register_theme_asset_sources, AssetSourcesPlugin,
     DEFAULT_THEME_MANIFEST_URL, USER_THEMES,
 };
+pub use theme::{set_theme, ActiveTheme, CardTheme, CardThemeLoader, ThemePlugin};
 pub use achievement_plugin::{AchievementPlugin, AchievementsResource, AchievementsScreen};
 pub use challenge_plugin::{
     challenge_progress_label, ChallengeAdvancedEvent, ChallengePlugin, CHALLENGE_UNLOCK_LEVEL,