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/AndroidManifest.xml

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools">
+
+    <uses-feature android:name="android.hardware.camera.any" />
+    <uses-permission android:name="android.permission.CAMERA" />
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+
+    <application
+        android:allowBackup="true"
+        android:label="USB摄像头"
+        android:theme="@style/Theme.AppCompat.NoActionBar">
+
+        <activity
+            android:name=".MainActivity"
+            android:exported="true"
+            android:screenOrientation="landscape"
+            android:keepScreenOn="true">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+</manifest>