feat: 添加 openim_common 依赖，重构环境配置和主页 URL 获取逻辑

openim_common/lib/openim_common.dart

@@ -0,0 +1,2 @@
+export 'src/config.dart';
+

openim_common/lib/src/config.dart

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+enum Environment {
+  production,
+}
+
+class Config {
+  static const Environment currentEnvironment = Environment.production;
+
+  static final Map<Environment, List<String>> _environmentHosts = {
+    Environment.production: [
+      'h5-im.imharry.work',
+    ],
+  };
+
+  static List<String> get environmentHosts {
+    return _environmentHosts[currentEnvironment] ?? const [];
+  }
+}

openim_common/pubspec.yaml

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+name: openim_common
+description: Build-server compatible configuration bridge for the H5 shell.
+publish_to: 'none'
+
+environment:
+  sdk: ">=3.6.0 <4.0.0"
+