feat: 添加 USB 摄像头连接功能

- 新增 Android USB 摄像头 APP (MJPEG 服务器)
- 电脑端支持 ADB 端口转发连接
- 修复 .gitignore 忽略 Android 文件

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

android-app/app/proguard-rules.pro

@@ -0,0 +1,10 @@
+# ProGuard rules for USB Webcam
+# Keep public class members
+-keep public class * {
+    public *;
+}
+
+# Keep Kotlin coroutines
+-keepnames class kotlinx.coroutines.internal.MainDispatcherFactory {}
+-keepnames class kotlinx.coroutines.CoroutineExceptionHandler {}
+-keep class kotlinx.coroutines.** { *; }