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/src/main/java/com/usbwebcam/MainActivity.kt

@@ -0,0 +1,101 @@
+package com.usbwebcam
+
+import android.Manifest
+import android.content.pm.PackageManager
+import android.os.Bundle
+import android.widget.Button
+import android.widget.TextView
+import android.widget.Toast
+import androidx.activity.result.contract.ActivityResultContracts
+import androidx.appcompat.app.AppCompatActivity
+import androidx.core.content.ContextCompat
+
+class MainActivity : AppCompatActivity() {
+    private var mjpegServer: MjpegServer? = null
+    private var cameraHelper: CameraHelper? = null
+
+    private val requestPermissionLauncher = registerForActivityResult(
+        ActivityResultContracts.RequestPermission()
+    ) { isGranted ->
+        if (isGranted) {
+            startCamera()
+        } else {
+            Toast.makeText(this, "需要相机权限", Toast.LENGTH_LONG).show()
+        }
+    }
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        setContentView(R.layout.activity_main)
+
+        val btnStart = findViewById<Button>(R.id.btn_start)
+        val btnStop = findViewById<Button>(R.id.btn_stop)
+        val btnCapture = findViewById<Button>(R.id.btn_capture)
+        val tvStatus = findViewById<TextView>(R.id.tv_status)
+
+        btnStart.setOnClickListener {
+            if (checkPermission()) {
+                startCamera()
+            }
+        }
+
+        btnStop.setOnClickListener {
+            stopCamera()
+        }
+
+        btnCapture.setOnClickListener {
+            cameraHelper?.captureAndSave()
+            Toast.makeText(this, "图片已保存到相册", Toast.LENGTH_SHORT).show()
+        }
+
+        // 自动启动
+        if (checkPermission()) {
+            startCamera()
+        }
+    }
+
+    private fun checkPermission(): Boolean {
+        return if (ContextCompat.checkSelfPermission(
+                this,
+                Manifest.permission.CAMERA
+            ) == PackageManager.PERMISSION_GRANTED
+        ) {
+            true
+        } else {
+            requestPermissionLauncher.launch(Manifest.permission.CAMERA)
+            false
+        }
+    }
+
+    private fun startCamera() {
+        cameraHelper = CameraHelper(this) { frame, _, _ ->
+            mjpegServer?.updateFrame(frame)
+        }
+
+        mjpegServer = MjpegServer(8080)
+        mjpegServer?.start {
+            runOnUiThread {
+                findViewById<TextView>(R.id.tv_status).text =
+                    "服务运行中端口: 8080IP: 无需IP (ADB模式)USB连接命令:adb forward tcp:8080 tcp:8080"
+                findViewById<Button>(R.id.btn_start).isEnabled = false
+                findViewById<Button>(R.id.btn_stop).isEnabled = true
+            }
+        }
+
+        cameraHelper?.start()
+    }
+
+    private fun stopCamera() {
+        cameraHelper?.stop()
+        mjpegServer?.stop()
+
+        findViewById<TextView>(R.id.tv_status).text = "服务已停止"
+        findViewById<Button>(R.id.btn_start).isEnabled = true
+        findViewById<Button>(R.id.btn_stop).isEnabled = false
+    }
+
+    override fun onDestroy() {
+        super.onDestroy()
+        stopCamera()
+    }
+}