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/theme_animation.rs

@@ -0,0 +1,22 @@
+//! Startup-recorded display hint for the theme system.
+//!
+//! The Nvidia/WebKit GPU quirk is detected once at process start (in `main()`,
+//! before the webview/GTK init). We record whether it was needed so the UI can
+//! warn that animated themes may raise CPU load on this setup — without
+//! re-probing the GPU later. Read via the `theme_animation_risk` command
+//! (`lib_commands::app_api::platform`).
+
+use std::sync::OnceLock;
+
+static NVIDIA_QUIRK_ACTIVE: OnceLock<bool> = OnceLock::new();
+
+/// Record the startup Nvidia-WebKit-quirk detection. Called once from `main()`.
+pub fn set_nvidia_quirk_active(active: bool) {
+    let _ = NVIDIA_QUIRK_ACTIVE.set(active);
+}
+
+/// Whether the Nvidia WebKit quirk was needed at startup. False when unrecorded
+/// (non-Linux, or GPU acceleration opted in via `PSYSONIC_WEBKIT_GPU_ACCEL`).
+pub(crate) fn nvidia_quirk_active() -> bool {
+    NVIDIA_QUIRK_ACTIVE.get().copied().unwrap_or(false)
+}