fix(download): expire the sticky error state

A download error stayed in the state flow forever: the dictionaries
section replayed it as a snackbar on every recomposition and screens
created later picked it up long after the failure, even while a newer
download was already running. Error states now decay to idle after the
same grace period as success, unless a newer state has replaced them.

app/src/main/java/com/example/research/feature/download/DownloadManager.kt

@@ -135,6 +135,10 @@ class DownloadManager(
                             }
 
                         }
+                        delay(TERMINAL_STATE_DURATION)
+                        if (downloadState.value is DownloadState.Error) {
+                            updateDownloadProgressState(DownloadState.Idle, 0f)
+                        }
                     }
                 } finally {
                     downloadJob = null
@@ -230,6 +234,10 @@ class DownloadManager(
                             }
 
                         }
+                        delay(TERMINAL_STATE_DURATION)
+                        if (downloadState.value is DownloadState.Error) {
+                            updateDownloadProgressState(DownloadState.Idle, 0f)
+                        }
                     }
                 } finally {
                     downloadJob = null