fix(android): make default receive path actually writable

Android emulated storage rejects hard-link commit even when canWrite()
reports success. Fall back to exclusive rename for publish, probe real
writes during folder validation, and label the default app downloads dir
clearly so it is not confused with shared system Downloads.

shared/src/androidMain/kotlin/com/vnidrop/app/core/FileSystemService.android.kt

@@ -22,14 +22,17 @@ private class AndroidFileSystemService(
 	private val context: Context,
 ) : FileSystemService {
 	override fun defaultReceiveFolder(): ReceiveFolder {
+		// App-specific external storage is always writable without SAF or
+		// legacy storage permissions. It is NOT the shared system Downloads
+		// gallery — that still requires "Choose folder" (tree URI).
 		val path = context
 			.getExternalFilesDir(Environment.DIRECTORY_DOWNLOADS)
 			?.absolutePath
-			?: (System.getProperty("java.io.tmpdir") ?: "/data/local/tmp/vnidrop-receive")
+			?: context.filesDir.resolve("Downloads").apply { mkdirs() }.absolutePath
 		return ReceiveFolder(
 			kind = ReceiveFolderKind.FileSystemPath,
 			value = path,
-			displayName = "Downloads",
+			displayName = "App downloads",
 		)
 	}
 
@@ -64,11 +67,27 @@ private class AndroidFileSystemService(
 		}
 	}
 
+	/**
+	 * Probe a real create/write/delete instead of [java.io.File.canWrite].
+	 *
+	 * Scoped storage often reports public directories as writable even when
+	 * the process cannot create files there. A probe matches what receive needs.
+	 */
 	private fun validatePath(path: String): FolderAccessStatus =
 		runCatching {
 			val directory = java.io.File(path)
-			if (!directory.exists()) directory.mkdirs()
-			if (directory.isDirectory && directory.canWrite()) FolderAccessStatus.Writable else FolderAccessStatus.Unavailable
+			if (!directory.exists() && !directory.mkdirs()) {
+				return FolderAccessStatus.Unavailable
+			}
+			if (!directory.isDirectory) return FolderAccessStatus.Unavailable
+			val probe = java.io.File(directory, ".vnidrop-write-test-${UUID.randomUUID()}")
+			try {
+				probe.outputStream().use { stream -> stream.write(1) }
+				if (!probe.exists()) return FolderAccessStatus.Unavailable
+				FolderAccessStatus.Writable
+			} finally {
+				probe.delete()
+			}
 		}.getOrDefault(FolderAccessStatus.Unavailable)
 
 	private fun validateTreeUri(value: String): FolderAccessStatus {