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/20260627125128_AddWorkflowRulesJson.cs

@@ -0,0 +1,35 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Backend.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddWorkflowRulesJson : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.Sql("""
+                ALTER TABLE "SiteSettings"
+                ADD COLUMN IF NOT EXISTS "WorkflowRulesJson" text NOT NULL DEFAULT '[]';
+                """);
+
+            migrationBuilder.UpdateData(
+                table: "SiteSettings",
+                keyColumn: "Id",
+                keyValue: 1,
+                column: "WorkflowRulesJson",
+                value: "[{\"key\":\"max_finalists_per_category\",\"label\":\"Finale Kandidat:innen pro Kategorie\",\"enabled\":true,\"limit\":4,\"mode\":\"block\",\"description\":\"Begrenzt, wie viele finale Kandidat:innen in einer Kategorie vorbereitet werden.\"},{\"key\":\"max_candidate_appearances\",\"label\":\"Kandidaturen pro Person\",\"enabled\":true,\"limit\":2,\"mode\":\"warn\",\"description\":\"Warnt oder blockiert, wenn dieselbe Person in zu vielen Kategorien final auftaucht.\"},{\"key\":\"max_winner_placements\",\"label\":\"Gewinnerpl\\u00E4tze pro Person\",\"enabled\":true,\"limit\":1,\"mode\":\"block\",\"description\":\"Verhindert oder warnt, wenn dieselbe Person mehrfach als Gewinner:in gesetzt wird.\"}]");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.Sql("""
+                ALTER TABLE "SiteSettings"
+                DROP COLUMN IF EXISTS "WorkflowRulesJson";
+                """);
+        }
+    }
+}