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/AdminSiteSettingsContracts.cs

@@ -14,6 +14,8 @@ public sealed record AdminSiteSettingsResponse(
     string ContactContent,
     string SponsorsUrl,
     string SponsorsContent,
+    string ShowactsUrl,
+    string ShowactsContent,
     IEnumerable<PublicSocialLinkDto> SocialLinks,
     IEnumerable<FaqItemDto> Faq);
 
@@ -29,6 +31,8 @@ public sealed record UpdateSiteSettingsRequest(
     string ContactContent,
     string SponsorsUrl,
     string SponsorsContent,
+    string ShowactsUrl,
+    string ShowactsContent,
     PublicSocialLinkDto[] SocialLinks,
     FaqItemDto[] Faq);
 
@@ -49,6 +53,24 @@ public sealed record AdminOperationalSettingsResponse(
     string MaintenanceTitle,
     string MaintenanceMessage);
 
+public sealed record AdminOptionalFeatureSettingsResponse(
+    bool ClipSubmissionsEnabled,
+    bool ClipReviewEnabled,
+    bool ClipAdminMenuVisible,
+    string ClipSubmissionDisabledMessage,
+    bool ShowactApplicationsEnabled,
+    string ShowactApplicationDisabledMessage,
+    bool SponsorsVisible);
+
+public sealed record UpdateOptionalFeatureSettingsRequest(
+    bool ClipSubmissionsEnabled,
+    bool ClipReviewEnabled,
+    bool ClipAdminMenuVisible,
+    string ClipSubmissionDisabledMessage,
+    bool ShowactApplicationsEnabled,
+    string ShowactApplicationDisabledMessage,
+    bool SponsorsVisible);
+
 public sealed record UpdateOperationalSettingsRequest(
     bool DemoLoginEnabled,
     string DemoLoginEmail,