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: SSO_URL (sso-mock service),
NEXT_PUBLIC_APP_URL (callback), SSO_APP_ID. .gitignore now ignores only
*.local overrides.

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

.env.staging

@@ -0,0 +1,8 @@
+# Mock SSO 服务地址（服务端用于 code 换取用户信息）
+SSO_URL="http://localhost:5501"
+
+# 本应用地址（作为 SSO 回调 app_url）
+NEXT_PUBLIC_APP_URL="http://localhost:4000"
+
+# 注册在 SSO 的应用 ID（app_id）
+SSO_APP_ID="app_demo_001"