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

.trellis/tasks/05-08-web-astro-react-tailwind-shadcn-ui/task.json

@@ -0,0 +1,26 @@
+{
+  "id": "web-astro-react-tailwind-shadcn-ui",
+  "name": "web-astro-react-tailwind-shadcn-ui",
+  "title": "Web端技术选型：Astro + React + Tailwind + shadcn/ui",
+  "description": "",
+  "status": "in_progress",
+  "dev_type": null,
+  "scope": null,
+  "package": null,
+  "priority": "P2",
+  "creator": "zl-q",
+  "assignee": "zl-q",
+  "createdAt": "2026-05-08",
+  "completedAt": null,
+  "branch": null,
+  "base_branch": "dev",
+  "worktree_path": null,
+  "commit": null,
+  "pr_url": null,
+  "subtasks": [],
+  "children": [],
+  "parent": null,
+  "relatedFiles": [],
+  "notes": "",
+  "meta": {}
+}