fix: probe source availability before showing download progress

Instant server failures (404/HTML error page) no longer flash a 0%
progress bar; the check mirrors the downloader's previous-day fallback
and tolerates transient network errors, which still go through the
downloader's retry path.

app/src/main/java/com/example/research/ReSearchApplication.kt

@@ -62,7 +62,8 @@ class ReSearchApplication : Application() {
 
         downloadManager = DownloadManager(
             dictionaryRepository = downloadDictionaryRepository,
-            unknownErrorMessage = getString(R.string.unknown_error)
+            unknownErrorMessage = getString(R.string.unknown_error),
+            sourceUnavailableMessage = getString(R.string.download_file_not_found)
         )
 
         dictionaryImportManager = DictionaryImportManager(