refactor(repo): consolidate openflare-server to root and move subprojects to internal/apps

- Merge all files inside openflare-server to the repository root directory.
- Relocate agent, relay, and flared subprojects from internal/ to internal/apps/.
- Combine docker-compose files and update build context paths to root.
- Update GitHub workflows and Dockerfiles to refer to new directories and package names.
- Rewrite Go package imports across all files.
- Resolve database renew test race condition and clean up docs.

frontend/.env.example

@@ -0,0 +1,11 @@
+# 前端 URL
+WAVELET_FRONTEND_URL=http://localhost:3000
+
+# 后端 URL
+WAVELET_BACKEND_URL=http://localhost:3000
+
+# Session Cookie
+WAVELET_SESSION_COOKIE_NAME=openflare_session_id
+
+# 环境
+NODE_ENV=production