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/hooks/useThemeAnimationRisk.ts

@@ -0,0 +1,46 @@
+import { useEffect, useState } from 'react';
+import { invoke } from '@tauri-apps/api/core';
+import { IS_LINUX } from '../utils/platform';
+
+/**
+ * Whether animated themes may raise CPU load on this setup — Linux with the
+ * Nvidia WebKit quirk active (recorded at startup) or compositing forced off.
+ * The store / theme cards show a warning on animated themes when this is true.
+ *
+ * Fetched once and cached for the process (read-once, per the Tauri-boundary
+ * rule). Always false off Linux, so the `invoke` is skipped there.
+ */
+let cached: boolean | null = null;
+
+export function useThemeAnimationRisk(): boolean {
+  const [risk, setRisk] = useState(cached ?? false);
+
+  useEffect(() => {
+    if (cached !== null) {
+      setRisk(cached);
+      return;
+    }
+    if (!IS_LINUX) {
+      cached = false;
+      return;
+    }
+    let alive = true;
+    const p = invoke<boolean>('theme_animation_risk');
+    // Guard the mocked-in-tests case where invoke isn't a real promise.
+    if (p && typeof (p as Promise<boolean>).then === 'function') {
+      (p as Promise<boolean>)
+        .then((v) => {
+          cached = !!v;
+          if (alive) setRisk(cached);
+        })
+        .catch(() => {
+          cached = false;
+        });
+    }
+    return () => {
+      alive = false;
+    };
+  }, []);
+
+  return risk;
+}