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/Contracts/PublicSeasonCategoryContracts.cs

@@ -4,10 +4,12 @@ public sealed record CandidateSummaryDto(
     int Id,
     string DisplayName,
     string ChannelSlug,
+    string ChannelUrl,
     string Platform,
     string? ClipUrl,
     string? ClipTitle,
-    string? ClipPlatform);
+    string? ClipPlatform,
+    string? ClipEmbedStatus);
 
 public sealed record PublicCategoryDetailDto(
     int Id,