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/index.css

@@ -0,0 +1,13 @@
+@layer theme, base, antd, components, utilities;
+@import "tailwindcss";
+
+html, body {
+    margin: 0;
+    padding: 0;
+    height: 100vh;
+    overflow: hidden;
+}
+
+#root {
+    height: 100%;
+}