feat(themes): warn about animated themes on high-CPU setups (#1020)

* feat(themes): warn about animated themes on high-CPU setups

Show a warning icon + tooltip on animated themes (those defining
@keyframes) in the store and in Your Themes, on Linux setups where
animation is costly — the Nvidia WebKit quirk is active or compositing
is forced off. The Nvidia detection already runs once at startup; its
result is recorded (theme_animation.rs) and read via a new
theme_animation_risk command — no GPU re-probe. Display-only; never
shown off Linux. The animated flag comes from the registry (store) or
the theme CSS (Your Themes).

* docs(themes): note the animated-theme warning PR in the Theme Store entry

src-tauri/src/lib_commands/app_api/mod.rs

@@ -23,6 +23,7 @@ pub(crate) use perf::performance_cpu_snapshot;
 pub(crate) use platform::{
     linux_wayland_gpu_font_tuning_active, linux_wayland_text_render_settings_available,
     set_linux_wayland_text_render_profile, set_linux_webkit_smooth_scrolling, set_window_decorations,
+    theme_animation_risk,
 };
 #[cfg(target_os = "linux")]
 pub(crate) use platform::{