perf(floating-bar): subscribe themeStore via selector

Both AppShell and PlayerBar read floatingPlayerBar via destructured
useThemeStore() without a selector, so every unrelated theme change
(accent, font, scheduler state, …) would force a re-render. Switch to
s => s.floatingPlayerBar so these components only re-render when the
toggle actually flips.

Follow-up to PR #216.

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

src/components/PlayerBar.tsx

@@ -101,7 +101,7 @@ export default function PlayerBar() {
     setUserRatingOverride: s.setUserRatingOverride,
   })));
   const { lastfmSessionKey } = useAuthStore();
-  const { floatingPlayerBar } = useThemeStore();
+  const floatingPlayerBar = useThemeStore(s => s.floatingPlayerBar);
   const [floatingStyle, setFloatingStyle] = useState<React.CSSProperties>({});
 
   useEffect(() => {