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/Domain/Sponsor.cs

@@ -0,0 +1,17 @@
+namespace Backend.Domain;
+
+public sealed class Sponsor
+{
+    public int Id { get; set; }
+    public int SeasonId { get; set; }
+    public Season Season { get; set; } = null!;
+    public string Name { get; set; } = string.Empty;
+    public string WebsiteUrl { get; set; } = string.Empty;
+    public string LogoUrl { get; set; } = string.Empty;
+    public string Description { get; set; } = string.Empty;
+    public string Tier { get; set; } = "Partner";
+    public int SortOrder { get; set; }
+    public bool IsVisible { get; set; } = true;
+    public DateTimeOffset CreatedAt { get; set; } = DateTimeOffset.UtcNow;
+    public DateTimeOffset? UpdatedAt { get; set; }
+}