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/lib/layout.shared.tsx

@@ -2,6 +2,7 @@ import type { BaseLayoutProps } from 'fumadocs-ui/layouts/shared';
 import { Heart } from 'lucide-react';
 import { Logo } from '@/components/logo';
 import { TelegramIcon } from '@/components/icons';
+import { DocsThemeSwitch } from '@/components/theme-switch';
 import { appName, productRepoUrl, telegramChannel, telegramChannelUrl, donateUrl, siteUrl } from './shared';
 import { getSiteMessages } from './site-i18n';
 
@@ -12,6 +13,9 @@ export function baseOptions(lang: string): BaseLayoutProps {
   const m = getSiteMessages(lang);
 
   return {
+    slots: {
+      themeSwitch: DocsThemeSwitch,
+    },
     nav: {
       title: (
         <span className="inline-flex items-center gap-2 font-semibold">