feat: in-app test file generator for upload testing

adb-pushed files are invisible to the app due to scoped storage, so add a
'Create 120 MB test file' button in the Downloads tab that writes the file
through the app's own uid (visible in its contribution set).

android/app/src/main/java/com/yarisobd/app/DashcamViewModel.kt

@@ -19,7 +19,9 @@ import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.asSharedFlow
 import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
 import retrofit2.Retrofit
 import retrofit2.converter.kotlinx.serialization.asConverterFactory
 import okhttp3.MediaType.Companion.toMediaType
@@ -257,6 +259,24 @@ class DashcamViewModel(application: Application) : AndroidViewModel(application)
         _downloadedFiles.value = files
     }
 
+    fun generateTestFile(sizeMb: Int = 120) {
+        viewModelScope.launch {
+            val f = File(getDownloadDir(), "test-upload-${sizeMb}mb.mp4")
+            try {
+                withContext(Dispatchers.IO) {
+                    f.outputStream().use { out ->
+                        val buf = ByteArray(1 shl 20)  // 1MB
+                        repeat(sizeMb) { out.write(buf) }
+                    }
+                }
+                scanDownloads()
+                _downloadEvents.emit("✓ Created test file: ${f.name} (${sizeMb} MB)")
+            } catch (e: Exception) {
+                _downloadEvents.emit("✗ Could not create test file: ${e.message}")
+            }
+        }
+    }
+
     fun deleteDownloaded(file: DownloadedFile) {
         _deletingNames.value = _deletingNames.value + file.name
         viewModelScope.launch {

android/app/src/main/java/com/yarisobd/app/DownloadsScreen.kt

@@ -67,6 +67,7 @@ fun DownloadsScreen(
     onCancel: (ActiveDownload) -> Unit,
     onUpload: (DownloadedFile) -> Unit,
     onCancelUpload: (ActiveDownload) -> Unit,
+    onGenerateTestFile: () -> Unit,
     onRefresh: () -> Unit,
 ) {
     val context = LocalContext.current
@@ -83,6 +84,15 @@ fun DownloadsScreen(
                 Text("Downloads", style = MaterialTheme.typography.titleLarge, color = MaterialTheme.colorScheme.primary)
                 Spacer(Modifier.height(4.dp))
                 Text("${files.size} files · Movies/yaris-obd/", style = MaterialTheme.typography.bodyMedium, color = MaterialTheme.colorScheme.onSurfaceVariant)
+                Spacer(Modifier.height(10.dp))
+                Button(
+                    onClick = onGenerateTestFile,
+                    modifier = Modifier.height(34.dp),
+                    shape = RoundedCornerShape(8.dp),
+                    contentPadding = PaddingValues(horizontal = 12.dp, vertical = 0.dp)
+                ) {
+                    Text("Create 120 MB test file", style = MaterialTheme.typography.bodySmall)
+                }
             }
         }
 

android/app/src/main/java/com/yarisobd/app/MainActivity.kt

@@ -135,6 +135,7 @@ fun YarisMainScreen(client: OkHttpClient) {
                     onCancel = { dashcamVm.cancelDownload(it) },
                     onUpload = { dashcamVm.uploadFile(it) },
                     onCancelUpload = { dashcamVm.cancelUpload(it) },
+                    onGenerateTestFile = { dashcamVm.generateTestFile() },
                     onRefresh = { dashcamVm.scanDownloads() },
                 )
                 2 -> SettingsScreen(client = client)