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/Candidate.cs

@@ -10,4 +10,10 @@ public sealed class Candidate
     public string DisplayName { get; set; } = string.Empty;
     public string ChannelSlug { get; set; } = string.Empty;
     public string Platform { get; set; } = "Twitch";
+    public string AcceptanceStatus { get; set; } = "open";
+    public string? AcceptanceNote { get; set; }
+    public string? ClipCompilationUrl { get; set; }
+    public string? ClipCompilationTitle { get; set; }
+    public string? ClipCompilationPlatform { get; set; }
+    public string ClipEmbedStatus { get; set; } = "unchecked";
 }