feat(web): rebuild web with Astro 6 + React 19 + Tailwind 4

Replace static HTML website with Astro SSG framework:
- Astro 6 + React 19 (client islands) + Tailwind CSS 4 + shadcn/ui
- i18n: zh/zh_Hant/en with URL prefix routing
- Pages: Landing, Features, Pricing, About, Privacy, Terms (3 locales)
- Responsive full-width layout with scroll reveal animations
- Cyber gradient theme with particle effects inspired by Kimi
- Features page: alternating layout with hexagram illustrations
- Legal pages: markdown rendering with side info card
- Language switcher preserves current page path
- Assets shared via symlinks to web/design/assets/ (no duplication)

Tech decisions documented in .trellis/spec/web/index.md
Task: .trellis/tasks/05-08-web-astro-react-tailwind-shadcn-ui

web/src/components/Navbar.astro

@@ -0,0 +1,44 @@
+---
+import { t, localePath, getLocaleLabel, type Locale } from '../i18n/utils';
+
+interface Props {
+  locale: Locale;
+}
+
+const { locale } = Astro.props;
+const currentPath = Astro.url.pathname.replace(new RegExp(`^/(zh|zh_Hant|en)`), '');
+const nav = t(locale, 'nav');
+const footer = t(locale, 'footer');
+const otherLocales: Locale[] = (['zh', 'zh_Hant', 'en'] as Locale[]).filter((l) => l !== locale);
+---
+
+<header class="w-full flex items-center justify-between h-16 md:h-20 px-5 md:px-20 border-b border-slate-200 bg-white sticky top-0 z-50">
+  <a href={localePath(locale, '/')} class="flex items-center gap-2 md:gap-3 shrink-0">
+    <img src="/images/logo.png" alt="MeiYao" class="w-8 h-8 md:w-9 md:h-9" />
+    <span class="text-slate-900 text-lg md:text-xl font-bold whitespace-nowrap">{footer.brandName}</span>
+  </a>
+
+  <nav class="flex items-center gap-4 md:gap-8">
+    <a href={localePath(locale, '/features')} class="hidden sm:block text-slate-600 text-sm hover:text-slate-900 whitespace-nowrap">{nav.features}</a>
+    <a href={localePath(locale, '/pricing')} class="hidden sm:block text-slate-600 text-sm hover:text-slate-900 whitespace-nowrap">{nav.pricing}</a>
+    <a href={localePath(locale, '/about')} class="hidden sm:block text-slate-600 text-sm hover:text-slate-900 whitespace-nowrap">{nav.about}</a>
+
+    <div class="relative group">
+      <button class="flex items-center gap-1 px-2 py-1.5 text-xs text-slate-600 rounded-lg border border-slate-200 bg-white hover:bg-slate-50 whitespace-nowrap">
+        {getLocaleLabel(locale)}
+        <svg class="w-3 h-3 text-slate-400" viewBox="0 0 12 12" fill="none"><path d="M3 5l3 3 3-3" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/></svg>
+      </button>
+      <div class="absolute right-0 top-full mt-1 bg-white border border-slate-200 rounded-lg shadow-lg py-1 opacity-0 invisible group-hover:opacity-100 group-hover:visible transition-all z-50">
+        {otherLocales.map((l) => (
+          <a href={localePath(l, currentPath)} class="block px-4 py-2 text-sm text-slate-600 hover:bg-slate-50 hover:text-slate-900 whitespace-nowrap">
+            {getLocaleLabel(l)}
+          </a>
+        ))}
+      </div>
+    </div>
+
+    <a href={localePath(locale, '/login')} class="bg-violet-600 text-white text-xs md:text-sm font-semibold px-4 md:px-5 py-2 md:py-2.5 rounded-lg hover:bg-violet-700 transition-colors whitespace-nowrap">
+      {nav.getStarted}
+    </a>
+  </nav>
+</header>