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/MediumFields.razor

@@ -1,4 +1,5 @@
 @using DeepDrftModels.Enums
+@using Microsoft.AspNetCore.Components.Forms
 
 @* The single dispatch point for medium-conditional form fields. All five upload/edit forms embed this
    one component; the @switch below is the ONLY place medium-specific form shape is decided. Adding a
@@ -26,7 +27,9 @@
                        Disabled="Disabled" />
             break;
         case ReleaseMedium.Session:
-            <SessionFields />
+            <SessionFields HeroImageFile="HeroImageFile"
+                           HeroImageFileChanged="HeroImageFileChanged"
+                           Disabled="Disabled" />
             break;
         case ReleaseMedium.Mix:
             <MixFields />
@@ -42,5 +45,9 @@
     [Parameter] public ReleaseType ReleaseType { get; set; } = ReleaseType.Single;
     [Parameter] public EventCallback<ReleaseType> ReleaseTypeChanged { get; set; }
 
+    // Session-only — the held hero-image file, uploaded after create. Ignored for Cut/Mix.
+    [Parameter] public IBrowserFile? HeroImageFile { get; set; }
+    [Parameter] public EventCallback<IBrowserFile?> HeroImageFileChanged { get; set; }
+
     [Parameter] public bool Disabled { get; set; }
 }