chore: restructure into monorepo and add open-source documentation

- Rename helix-server -> server and helix-web -> web
- Remove helix-api (bruno collection and docs site)
- Merge per-module .gitignore files into the root .gitignore
- Add README, AGENTS, CONTRIBUTING and MIT LICENCE
- Add full-stack docker-compose (postgres, redis, server, web via Caddy)
  with web Dockerfile, Caddyfile and Compose server config
- Keep the Gradle wrapper jar tracked after the .gitignore merge
- Copy gradle.properties into the server Docker build image

web/src/page/not-found/index.tsx

@@ -0,0 +1,30 @@
+import { Link } from "react-router"
+
+/**
+ * General page not found page.
+ * @constructor
+ */
+export default function NotFoundPage() {
+  return (
+    <div className="min-h-screen bg-gradient-to-br from-blue-50 to-indigo-100 flex items-center justify-center p-4">
+      <div className="absolute top-0 left-0 w-72 h-72 bg-blue-200 rounded-full mix-blend-multiply filter blur-xl opacity-70"></div>
+      <div className="absolute top-0 right-0 w-72 h-72 bg-purple-200 rounded-full mix-blend-multiply filter blur-xl opacity-70"></div>
+      <div className="absolute bottom-0 left-1/2 w-72 h-72 bg-pink-200 rounded-full mix-blend-multiply filter blur-xl opacity-70"></div>
+
+      <div className="text-center bg-white p-10 lg:p-14 rounded-xl shadow-lg md:shadow-xl max-w-xl w-full">
+        <h1 className="text-7xl lg:text-8xl text-amber-500 m-0 font-bold tracking-wider">404</h1>
+        <h2 className="text-3xl md:text-4xl text-gray-700 mt-4 mb-6">找不到页面</h2>
+        <p className="text-lg text-gray-600 leading-relaxed mb-9">
+          找不到您需要的页面，您是否输入了错误的地址？
+        </p>
+        <Link
+          to="/"
+          className="inline-block bg-blue-600 text-white py-3 px-7 rounded-lg no-underline
+                     text-lg font-semibold transition-all duration-300 ease-in-out
+                     shadow-md shadow-blue-500/20 hover:bg-blue-700 hover:-translate-y-0.5 hover:shadow-lg hover:shadow-blue-500/30">
+          返回主页
+        </Link>
+      </div>
+    </div>
+  )
+}