接入Gitea Actions蓝绿发布：API_KEY构建时注入、新增Dockerfile

- src/config.js 的 API_KEY 改为从构建时环境变量 TINODE_API_KEY 注入，
  不再依赖官方demo默认值；未设置时保留原demo key作为本地快速测试的兜底
- webpack.config.js 新增 DefinePlugin 完成注入，并自动加载(gitignored的)
  .env.local，开发者本地调试不需要每次手动export
- 新增 Dockerfile(node构建+nginx serve) 和 docker/nginx.conf
- 新增 .gitea/workflows/deploy.yml：手动触发的蓝绿发布流程

src/config.js

@@ -5,8 +5,11 @@ export const BASE_APP_NAME = 'TinodeWeb';
 // Name of this application, used in the User-Agent.
 export const APP_NAME = BASE_APP_NAME + '/' + (PACKAGE_VERSION || '0.25');
 
-// API key. Use https://github.com/tinode/chat/tree/master/keygen to generate your own
-export const API_KEY = 'AQEAAAABAAD_rAp4DJh05a1HAwFT3A6K';
+// API key. Use https://github.com/tinode/chat/tree/master/keygen to generate your own.
+// Injected at build time via webpack DefinePlugin from the TINODE_API_KEY env var
+// (see webpack.config.js and .env.local). Falls back to the public sample key when unset,
+// which only works against a backend still using the matching sample API_KEY_SALT.
+export const API_KEY = process.env.TINODE_API_KEY || 'AQEAAAABAAD_rAp4DJh05a1HAwFT3A6K';
 
 // The array of possible hosts to connect to.
 export const KNOWN_HOSTS = {hosted: 'web.tinode.co', local: 'localhost:6060'};