fix(mining-app): APK 安装后调用 finishAffinity() 退出旧应用，与 mobile-app 保持一致

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-app/android/app/src/main/kotlin/com/rwadurian/mining_app/MainActivity.kt

@@ -70,6 +70,8 @@ class MainActivity : FlutterActivity() {
             }
 
             startActivity(intent)
+            // 退出当前应用，让系统安装器可以替换 APK
+            finishAffinity()
             true
         } catch (e: Exception) {
             e.printStackTrace()