chore: add per-environment env config (参考 uirefbase)

Commit .env.development/.staging/.production and track env files in git
(matching uirefbase convention). No secrets: only NEXT_PUBLIC_BASE_PATH
and NEXT_PUBLIC_SHARED_API_PATH (basis backend). .gitignore now ignores
only *.local overrides.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.production

@@ -0,0 +1,5 @@
+# base url（部署子路径，无则留空）
+NEXT_PUBLIC_BASE_PATH=""
+
+# 公共服务接口地址（后端 basis 服务，集群内 svc 名，与 uirefbase 生产一致）
+NEXT_PUBLIC_SHARED_API_PATH="http://basis-app-svc:6610"