mobile: 提交 android/ios 平台脚手架与打包配置（B-58）

- flutter create 生成的 android/ ios/ 脚手架入库，服务器/使用者无需再装 Flutter
- AndroidManifest：补 INTERNET/录音/存储权限，应用名改「畅联」，开 usesCleartextTraffic（内网 http/ws）
- iOS Info.plist：麦克风/相册用途说明、ATS 放行内网明文、显示名「畅联」
- gradle：阿里云镜像优先（本网络 maven.google.com 不可达）；统一 Java/Kotlin 编译目标 17；
  抬升 livekit_client compileSdk 到 36 以满足 flutter_webrtc 的 AAR 校验
- README 更新为「直接打包」流程

mobile/.metadata

@@ -0,0 +1,33 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: "6b182d2c7585eba26d4edce0f97630effd256c33"
+  channel: "stable"
+
+project_type: app
+
+# Tracks metadata for the flutter migrate command
+migration:
+  platforms:
+    - platform: root
+      create_revision: 6b182d2c7585eba26d4edce0f97630effd256c33
+      base_revision: 6b182d2c7585eba26d4edce0f97630effd256c33
+    - platform: android
+      create_revision: 6b182d2c7585eba26d4edce0f97630effd256c33
+      base_revision: 6b182d2c7585eba26d4edce0f97630effd256c33
+    - platform: ios
+      create_revision: 6b182d2c7585eba26d4edce0f97630effd256c33
+      base_revision: 6b182d2c7585eba26d4edce0f97630effd256c33
+
+  # User provided section
+
+  # List of Local paths (relative to this file) that should be
+  # ignored by the migrate tool.
+  #
+  # Files that are not part of the templates will be ignored by default.
+  unmanaged_files:
+    - 'lib/main.dart'
+    - 'ios/Runner.xcodeproj/project.pbxproj'