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/.env.example

@@ -0,0 +1,14 @@
+# Backend base URL
+VITE_API_BASE_URL=/api
+# Microsoft Entra ID Client ID
+VITE_MSAL_CLIENT_ID=msal-client-id
+# Microsoft Entra ID Tenant ID
+VITE_MSAL_TENANT_ID=msal-tenant-id
+# Default region of user. For more regions, please refer to ISO 3166-1 alpha-2 code
+#   Examples
+#     - GB: United Kingdom
+#     - US: United States
+#     - DE: Germany
+VITE_DEFAULT_REGION_ABBREVIATION=GB
+# Application title
+VITE_APP_TITLE='OnixByte Technology Co., Ltd'