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/astro.config.mjs

@@ -0,0 +1,18 @@
+// @ts-check
+import { defineConfig } from 'astro/config';
+import react from '@astrojs/react';
+import tailwindcss from '@tailwindcss/vite';
+
+export default defineConfig({
+  integrations: [react()],
+  i18n: {
+    locales: ['zh', 'zh_Hant', 'en'],
+    defaultLocale: 'zh',
+    routing: {
+      prefixDefaultLocale: true,
+    },
+  },
+  vite: {
+    plugins: [tailwindcss()],
+  },
+});