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/pages/zh_Hant/index.astro

@@ -0,0 +1,13 @@
+---
+import MarketingLayout from '../../layouts/Marketing.astro';
+import Hero from '../../components/Hero.astro';
+import Showcase from '../../components/Showcase.astro';
+import CtaSection from '../../components/CtaSection.astro';
+
+const locale = 'zh_Hant' as const;
+---
+<MarketingLayout locale={locale}>
+  <Hero locale={locale} />
+  <Showcase locale={locale} />
+  <CtaSection locale={locale} />
+</MarketingLayout>