feat(cms): compose Session hero image into the upload form (8.F)

Session upload now carries a deferred hero-image input; the submit handler
creates the release then POSTs the held hero to the existing resource-addressed
endpoint. Hero is optional with a non-blocking warn-if-missing gate. The
per-row hero upload in CmsSessionBrowser remains the replace/correct path.

DeepDrftManager/Components/Pages/Tracks/AlbumHeaderFields.razor

@@ -66,6 +66,8 @@
 
     <MediumFields @bind-Medium="MediumBinding"
                   @bind-ReleaseType="ReleaseTypeBinding"
+                  HeroImageFile="HeroImageFile"
+                  HeroImageFileChanged="HeroImageFileChanged"
                   Disabled="Disabled" />
 </MudPaper>
 
@@ -85,6 +87,11 @@
     [Parameter] public IBrowserFile? SelectedImageFile { get; set; }
     [Parameter] public EventCallback<IBrowserFile?> SelectedImageFileChanged { get; set; }
 
+    // Session-only — the held hero-image file, threaded through MediumFields to SessionFields.
+    // Ignored for Cut/Mix media. The parent (BatchUpload) owns it and uploads it after create.
+    [Parameter] public IBrowserFile? HeroImageFile { get; set; }
+    [Parameter] public EventCallback<IBrowserFile?> HeroImageFileChanged { get; set; }
+
     // BatchEdit only: when set (and no new file picked), preview the release's current cover.
     // The parent nulls this to drop the preview when the admin clears the existing cover.
     [Parameter] public string? ExistingImagePath { get; set; }