fix: register-new-then-remove-old in ReplaceTrackAudioAsync; replace wording in timeout messages; doc comment on ExistingTrackCount

On partial failure the old path deleted the original audio before
confirming the new write succeeded. Now: load old extension, register
new audio first (original untouched on failure), then clean up stale
backing file only on success and only when extension changed.

DeepDrftManager/Components/Pages/Tracks/BatchEdit.razor

@@ -59,6 +59,9 @@
            single-track medium, mirroring BatchUpload's same-named collapse. Cut keeps the full list. *@
         <MudGrid>
             <MudItem xs="12" md="5">
+                @* ExistingTrackCount counts edit-session persisted rows (Id.HasValue), not authoritative
+                   live release count — acceptable because this gate only hides a UI control; the
+                   TrySoftDeleteEmptyReleaseAsync backstop remains the authoritative guard. *@
                 <BatchTrackList Tracks="_tracks"
                                 @bind-SelectedIndex="_selectedIndex"
                                 Disabled="_saving"

DeepDrftManager/Services/CmsTrackService.cs

@@ -272,12 +272,12 @@ public class CmsTrackService : ICmsTrackService
                     _owner._logger.LogWarning("{Operation} stalled — no progress for {IdleSeconds}s; aborting.",
                         operationLabel, _owner._uploadIdleTimeout.TotalSeconds);
                     return LargeBodySendResult.Fail(
-                        $"Upload stalled — no data transferred for {_owner._uploadIdleTimeout.TotalSeconds:0}s. Please retry.");
+                        $"{operationLabel} stalled — no data transferred for {_owner._uploadIdleTimeout.TotalSeconds:0}s. Please retry.");
                 }
                 _owner._logger.LogWarning("{Operation} timed out waiting for server response after {ResponseSeconds}s.",
                     operationLabel, _owner._uploadResponseTimeout.TotalSeconds);
                 return LargeBodySendResult.Fail(
-                    $"Upload timed out waiting for the server to respond after {_owner._uploadResponseTimeout.TotalSeconds:0}s. Please retry.");
+                    $"{operationLabel} timed out waiting for the server to respond after {_owner._uploadResponseTimeout.TotalSeconds:0}s. Please retry.");
             }
             catch (Exception ex)
             {