Improve admin candidate modal UX and add clip menu visibility toggle

- Widen AdminCandidateEditorModal to size lg for better readability
- Rename "Clip-Compilation" section to "Clip / Compilation", update copy to reflect single clips too, drop upload hint and Clip-Plattform field, rename label to "Link"
- Fix NativeSelect dropdown clipping inside overflow-y-auto modals by teleporting the menu to body with fixed positioning, flip-up logic, and dynamic maxHeight capped to viewport
- Add ClipAdminMenuVisible setting (backend domain, contracts, endpoint, migration) with matching frontend types, defaults, form wiring, and toggle in the Clip-Workflow modal — hides the Clips nav item from the admin sidebar when disabled

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Backend/Endpoints/PublicEndpoints.cs

@@ -22,6 +22,10 @@ public static partial class PublicEndpoints
             .WithName("GetWinnerArchive")
             .WithOpenApi();
 
+        group.MapGet("/seasons/{year:int}/sponsors", GetSponsors)
+            .WithName("GetPublicSponsors")
+            .WithOpenApi();
+
         group.MapGet("/seasons/{year:int}/me", GetUserParticipation)
             .WithName("GetUserParticipation")
             .WithOpenApi();
@@ -41,6 +45,11 @@ public static partial class PublicEndpoints
             .WithName("CreateClip")
             .WithOpenApi();
 
+        group.MapPost("/showacts", CreateShowactApplication)
+            .RequireRateLimiting(Backend.Common.ApplicationDefaults.PublicWriteRateLimitPolicy)
+            .WithName("CreateShowactApplication")
+            .WithOpenApi();
+
         return app;
     }
 }