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/Common/SeasonMappings.cs

@@ -184,5 +184,6 @@ public static class SeasonMappings
         new FooterLinkDto("imprint", "Impressum", settings.ImprintUrl, settings.ImprintContent),
         new FooterLinkDto("contact", "Kontakt", settings.ContactUrl, settings.ContactContent),
         new FooterLinkDto("sponsors", "Sponsoren & Partner", settings.SponsorsUrl, settings.SponsorsContent),
+        new FooterLinkDto("showacts", "Showacts", settings.ShowactsUrl, settings.ShowactsContent),
     ];
 }