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/PublicClipEndpoints.cs

@@ -28,6 +28,22 @@ public static partial class PublicEndpoints
             return Results.BadRequest(new { message = "Only Twitch or YouTube clip links are supported." });
         }
 
+        var siteSettings = await db.SiteSettings
+            .AsNoTracking()
+            .FirstOrDefaultAsync(item => item.Id == 1, context.RequestAborted);
+        if (siteSettings is null)
+        {
+            return Results.Problem("Site settings are missing.");
+        }
+
+        if (!siteSettings.ClipSubmissionsEnabled)
+        {
+            var message = string.IsNullOrWhiteSpace(siteSettings.ClipSubmissionDisabledMessage)
+                ? "Clip-Einreichungen sind aktuell geschlossen."
+                : siteSettings.ClipSubmissionDisabledMessage;
+            return Results.BadRequest(new { message });
+        }
+
         var season = await db.Seasons.FirstOrDefaultAsync(item => item.Year == request.Year);
         var clipSeasonResolution = EnsurePublicWriteSeason(season, "nomination");
         if (clipSeasonResolution.Result is not null)