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/PublicNominationEndpoints.cs

@@ -54,6 +54,17 @@ public static partial class PublicEndpoints
             return Results.BadRequest(new { message = "A valid http(s) stream link is required." });
         }
 
+        var settings = await db.SiteSettings.AsNoTracking().FirstOrDefaultAsync(item => item.Id == 1);
+        var linkBlacklist = NominationLinkBlacklistSettings.Read(settings);
+        var blacklistedStreamUrl = submittedNominations
+            .Select(item => item.StreamUrl)
+            .FirstOrDefault(item => NominationLinkBlacklistSettings.IsBlocked(item, linkBlacklist));
+
+        if (blacklistedStreamUrl is not null)
+        {
+            return Results.BadRequest(new { message = "Dieser Link kann nicht nominiert werden. Bitte reiche einen direkten Kanal- oder Profil-Link ein." });
+        }
+
         var category = await db.Categories
             .Include(item => item.Season)
             .FirstOrDefaultAsync(item => item.Id == request.CategoryId && item.Season.Year == request.Year);