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/package.json

@@ -0,0 +1,25 @@
+{
+  "name": "eryao-web",
+  "type": "module",
+  "version": "0.0.1",
+  "engines": {
+    "node": ">=22.12.0"
+  },
+  "scripts": {
+    "dev": "astro dev",
+    "build": "astro build",
+    "preview": "astro preview",
+    "astro": "astro"
+  },
+  "dependencies": {
+    "@astrojs/react": "^5.0.4",
+    "@tailwindcss/vite": "^4.3.0",
+    "@types/react": "^19.2.14",
+    "@types/react-dom": "^19.2.3",
+    "astro": "^6.3.1",
+    "marked": "^18.0.3",
+    "react": "^19.2.6",
+    "react-dom": "^19.2.6",
+    "tailwindcss": "^4.3.0"
+  }
+}