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/Migrations/20260627135729_AddNominationLinkBlacklist.cs

@@ -0,0 +1,36 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Backend.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddNominationLinkBlacklist : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<string>(
+                name: "NominationLinkBlacklistJson",
+                table: "SiteSettings",
+                type: "text",
+                nullable: false,
+                defaultValue: "[]");
+
+            migrationBuilder.UpdateData(
+                table: "SiteSettings",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "NominationLinkBlacklistJson",
+                value: "[{\"Url\":\"https://kick.com/\"},{\"Url\":\"https://www.twitch.tv/\"},{\"Url\":\"https://www.youtube.com/\"}]");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "NominationLinkBlacklistJson",
+                table: "SiteSettings");
+        }
+    }
+}