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/AdminModerationEndpoints.cs

@@ -20,6 +20,18 @@ public static partial class AdminModerationEndpoints
             .AddEndpointFilter((context, next) => AdminEndpointConventions.RequireWritePermission(context, next, Backend.Security.AdminPermissionCatalog.Nominations))
             .WithName("RejectAdminNomination")
             .WithOpenApi();
+        group.MapGet("/nominations/link-blacklist", GetNominationLinkBlacklist)
+            .AddEndpointFilter((context, next) => AdminEndpointConventions.RequirePermission(context, next, Backend.Security.AdminPermissionCatalog.Nominations))
+            .WithName("GetAdminNominationLinkBlacklist")
+            .WithOpenApi();
+        group.MapPut("/nominations/link-blacklist", UpdateNominationLinkBlacklist)
+            .AddEndpointFilter((context, next) => AdminEndpointConventions.RequireWritePermission(context, next, Backend.Security.AdminPermissionCatalog.Nominations))
+            .WithName("UpdateAdminNominationLinkBlacklist")
+            .WithOpenApi();
+        group.MapPost("/nominations/link-blacklist", AddNominationLinkBlacklistEntry)
+            .AddEndpointFilter((context, next) => AdminEndpointConventions.RequireWritePermission(context, next, Backend.Security.AdminPermissionCatalog.Nominations))
+            .WithName("AddAdminNominationLinkBlacklistEntry")
+            .WithOpenApi();
         group.MapGet("/risk-flags", GetRiskFlags)
             .AddEndpointFilter((context, next) => AdminEndpointConventions.RequirePermission(context, next, Backend.Security.AdminPermissionCatalog.Risk))
             .WithName("GetAdminRiskFlags")