feat: add persistent light/dark mode toggle

- Add CSS custom property token system to globals.css (dark + light variants)
- Add anti-FOUC inline script in layout.tsx head to apply stored theme before hydration
- Create useTheme hook (src/hooks/useTheme.ts) backed by usePersistentState
- Add Sun/Moon toggle button to LeftToolbar above Settings
- Wire useTheme into ThreePanelLayout and pass theme/onThemeToggle to LeftToolbar
- Migrate hardcoded hex values to CSS vars in LeftToolbar and ThreePanelLayout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/hooks/useTheme.ts

@@ -0,0 +1,28 @@
+"use client";
+
+import { useEffect } from 'react';
+import { usePersistentState } from './usePersistentState';
+
+export type Theme = 'dark' | 'light';
+
+function getInitialTheme(): Theme {
+  if (typeof document === 'undefined') {
+    return 'dark';
+  }
+
+  return document.documentElement.getAttribute('data-theme') === 'light' ? 'light' : 'dark';
+}
+
+export function useTheme(): [Theme, () => void] {
+  const [theme, setTheme] = usePersistentState<Theme>('ui.theme', getInitialTheme());
+
+  useEffect(() => {
+    document.documentElement.setAttribute('data-theme', theme);
+  }, [theme]);
+
+  const toggleTheme = () => {
+    setTheme((prev) => (prev === 'dark' ? 'light' : 'dark'));
+  };
+
+  return [theme, toggleTheme];
+}