新增扫码功能

- 新增 `ScanQrScreen.kt` 文件，用于实现二维码扫描界面。
- 使用 CameraX 和 ML Kit Barcode Scanning 实现二维码识别。
- 请求相机权限，并在权限被拒绝时显示提示信息。
- 扫描成功后，通过 `savedStateHandle` 将结果返回给上一个界面并关闭当前屏幕。

app/src/main/java/com/aiosman/ravenow/ui/scan/ScanQrScreen.kt

@@ -0,0 +1,171 @@
+package com.aiosman.ravenow.ui.scan
+
+import android.Manifest
+import android.widget.Toast
+import androidx.activity.compose.rememberLauncherForActivityResult
+import androidx.activity.result.contract.ActivityResultContracts
+import androidx.camera.core.CameraSelector
+import androidx.camera.core.ImageAnalysis
+import androidx.camera.core.Preview
+import androidx.camera.lifecycle.ProcessCameraProvider
+import androidx.camera.view.PreviewView
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.Button
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalContext
+import androidx.compose.ui.platform.LocalLifecycleOwner
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.viewinterop.AndroidView
+import androidx.core.content.ContextCompat
+import com.aiosman.ravenow.LocalNavController
+import com.google.mlkit.vision.barcode.common.Barcode
+import com.google.mlkit.vision.barcode.BarcodeScannerOptions
+import com.google.mlkit.vision.barcode.BarcodeScanning
+import com.google.mlkit.vision.common.InputImage
+import java.util.concurrent.Executors
+
+@Composable
+fun ScanQrScreen() {
+    val context = LocalContext.current
+    val lifecycleOwner = LocalLifecycleOwner.current
+    val navController = LocalNavController.current
+
+    var cameraGranted by remember { mutableStateOf<Boolean?>(null) }
+    var hasResult by remember { mutableStateOf(false) }
+
+    val requestPermissionLauncher = rememberLauncherForActivityResult(
+        contract = ActivityResultContracts.RequestPermission(),
+        onResult = { granted ->
+            cameraGranted = granted
+        }
+    )
+
+    LaunchedEffect(Unit) {
+        requestPermissionLauncher.launch(Manifest.permission.CAMERA)
+    }
+
+    val scanner = remember {
+        val options = BarcodeScannerOptions.Builder()
+            .setBarcodeFormats(Barcode.FORMAT_QR_CODE)
+            .build()
+        BarcodeScanning.getClient(options)
+    }
+
+    when (cameraGranted) {
+        null -> {
+            // 等待权限结果
+            Box(modifier = Modifier.fillMaxSize().background(Color.Black))
+        }
+        false -> {
+            // 权限被拒绝
+            Column(
+                modifier = Modifier.fillMaxSize().background(Color.Black),
+                verticalArrangement = Arrangement.Center,
+                horizontalAlignment = Alignment.CenterHorizontally
+            ) {
+                Text(text = "需要相机权限以扫码", color = Color.White, modifier = Modifier.padding(16.dp))
+                Button(onClick = { navController.popBackStack() }) {
+                    Text(text = "返回")
+                }
+            }
+        }
+        true -> {
+            Box(modifier = Modifier.fillMaxSize().background(Color.Black)) {
+                AndroidView(
+                    modifier = Modifier.fillMaxSize(),
+                    factory = { ctx ->
+                        PreviewView(ctx).apply {
+                            this.scaleType = PreviewView.ScaleType.FILL_CENTER
+                        }
+                    },
+                    update = { previewView ->
+                        val cameraProviderFuture = ProcessCameraProvider.getInstance(context)
+                        cameraProviderFuture.addListener({
+                            val cameraProvider = cameraProviderFuture.get()
+
+                            val preview = Preview.Builder().build().also { p ->
+                                p.setSurfaceProvider(previewView.surfaceProvider)
+                            }
+
+                            val analysis = ImageAnalysis.Builder()
+                                .setBackpressureStrategy(ImageAnalysis.STRATEGY_KEEP_ONLY_LATEST)
+                                .build()
+                                .also { imageAnalysis ->
+                                    val executor = Executors.newSingleThreadExecutor()
+                                    imageAnalysis.setAnalyzer(executor) { imageProxy ->
+                                        val mediaImage = imageProxy.image
+                                        if (mediaImage == null) {
+                                            imageProxy.close()
+                                            return@setAnalyzer
+                                        }
+                                        val image = InputImage.fromMediaImage(
+                                            mediaImage,
+                                            imageProxy.imageInfo.rotationDegrees
+                                        )
+                                        scanner.process(image)
+                                            .addOnSuccessListener { barcodes ->
+                                                if (!hasResult && !barcodes.isNullOrEmpty()) {
+                                                    val first = barcodes.firstOrNull()
+                                                    val rawValue = first?.rawValue ?: first?.displayValue
+                                                    if (!rawValue.isNullOrBlank()) {
+                                                        hasResult = true
+                                                        navController.previousBackStackEntry
+                                                            ?.savedStateHandle
+                                                            ?.set("scan_result", rawValue)
+                                                        Toast.makeText(context, rawValue, Toast.LENGTH_SHORT).show()
+                                                        navController.popBackStack()
+                                                    }
+                                                }
+                                            }
+                                            .addOnFailureListener {
+                                                // 忽略单帧失败
+                                            }
+                                            .addOnCompleteListener {
+                                                imageProxy.close()
+                                            }
+                                    }
+                                }
+
+                            try {
+                                cameraProvider.unbindAll()
+                                cameraProvider.bindToLifecycle(
+                                    lifecycleOwner,
+                                    CameraSelector.DEFAULT_BACK_CAMERA,
+                                    preview,
+                                    analysis
+                                )
+                            } catch (_: Exception) {
+                                // 绑定失败，忽略
+                            }
+                        }, ContextCompat.getMainExecutor(context))
+                    }
+                )
+
+                // 简单文案提示
+                Text(
+                    text = "对准二维码进行扫描",
+                    color = Color.White,
+                    modifier = Modifier
+                        .align(Alignment.BottomCenter)
+                        .padding(bottom = 32.dp)
+                )
+            }
+        }
+    }
+}
+
+