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

.gitignore

@@ -197,6 +197,15 @@ unlinked_spec.ds
 # IDE
 .idea/
 
+# Codex local workspace helpers and runtime state
+.codex/
+
+# Web (Astro)
+node_modules/
+.astro/
+web/dist/
+web/package-lock.json
+
 # Android related
 **/android/**/gradle-wrapper.jar
 .gradle/