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/build/intermediates/apk/debug/output-metadata.json

@@ -0,0 +1,21 @@
+{
+  "version": 3,
+  "artifactType": {
+    "type": "APK",
+    "kind": "Directory"
+  },
+  "applicationId": "com.usbwebcam",
+  "variantName": "debug",
+  "elements": [
+    {
+      "type": "SINGLE",
+      "filters": [],
+      "attributes": [],
+      "versionCode": 1,
+      "versionName": "1.0",
+      "outputFile": "app-debug.apk"
+    }
+  ],
+  "elementType": "File",
+  "minSdkVersionForDexing": 24
+}