fix(docs): restore theme switch without runtime warnings

Move html/body shell and global css to root app layout to avoid hydration/script warnings from nested document nodes. Disable provider theme injection and add a custom script-free theme switch in shared layout slots.

Also migrate docs search static client initializer to ZBSearch (initDB), add zbsearch dependency, and align docs lint tooling with ESLint 9 compatibility so npm run lint passes.

docs/components/theme-switch.tsx

@@ -0,0 +1,83 @@
+'use client';
+
+import { Moon, Sun } from 'lucide-react';
+import { useEffect, useMemo, useState } from 'react';
+import type { ComponentProps } from 'react';
+import { cn } from '@/lib/cn';
+
+type ThemeMode = 'light-dark' | 'light-dark-system';
+type ThemePref = 'light' | 'dark' | 'system';
+
+const STORAGE_KEY = 'docs-theme';
+
+function getStoredTheme(): ThemePref {
+  if (typeof window === 'undefined') return 'system';
+  const raw = window.localStorage.getItem(STORAGE_KEY);
+  return raw === 'light' || raw === 'dark' || raw === 'system' ? raw : 'system';
+}
+
+function getResolvedTheme(theme: ThemePref): 'light' | 'dark' {
+  if (theme !== 'system') return theme;
+  if (typeof window === 'undefined') return 'light';
+  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+}
+
+function applyTheme(theme: ThemePref): void {
+  if (typeof document === 'undefined') return;
+  const resolved = getResolvedTheme(theme);
+  const root = document.documentElement;
+  root.classList.toggle('dark', resolved === 'dark');
+  root.style.colorScheme = resolved;
+}
+
+export function DocsThemeSwitch({
+  className,
+  mode = 'light-dark-system',
+  ...props
+}: {
+  className?: string;
+  mode?: ThemeMode;
+} & Omit<ComponentProps<'div'>, 'children'>) {
+  const [theme, setTheme] = useState<ThemePref>(getStoredTheme);
+
+  useEffect(() => {
+    window.localStorage.setItem(STORAGE_KEY, theme);
+    applyTheme(theme);
+  }, [theme]);
+
+  useEffect(() => {
+    if (theme !== 'system') return;
+    const media = window.matchMedia('(prefers-color-scheme: dark)');
+    const update = () => applyTheme('system');
+    media.addEventListener('change', update);
+    return () => media.removeEventListener('change', update);
+  }, [theme]);
+
+  const resolved = useMemo(() => getResolvedTheme(theme), [theme]);
+
+  const nextTheme = () => {
+    if (mode === 'light-dark') return resolved === 'dark' ? 'light' : 'dark';
+    if (theme === 'light') return 'dark';
+    if (theme === 'dark') return 'system';
+    return resolved === 'dark' ? 'light' : 'dark';
+  };
+
+  const label =
+    mode === 'light-dark-system'
+      ? `Switch theme (current: ${theme})`
+      : `Switch to ${resolved === 'dark' ? 'light' : 'dark'} mode`;
+
+  return (
+    <div className={cn('inline-flex', className)} {...props}>
+      <button
+        type="button"
+        aria-label={label}
+        title={label}
+        onClick={() => setTheme(nextTheme())}
+        className="inline-flex size-8 items-center justify-center rounded-lg text-fd-muted-foreground transition-colors hover:bg-fd-accent hover:text-fd-accent-foreground"
+      >
+        {resolved === 'dark' ? <Moon className="size-4" /> : <Sun className="size-4" />}
+      </button>
+    </div>
+  );
+}