Wire ShareButton into list/note/calendar pages; fix nav and calendar select labels

- Add ShareButton to list detail, note editor, and calendar event panel
- Add Settings gear icon to AppNav (core page not in module registry)
- Fix calendar event dialog showing raw UUID: pass explicit children to
  SelectValue so labels display before SelectContent mounts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/app-nav.tsx

@@ -1,4 +1,5 @@
 import Link from "next/link";
+import { Settings } from "lucide-react";
 import { getRegistry } from "@/modules/_core/registry";
 
 export function AppNav() {
@@ -19,6 +20,13 @@ export function AppNav() {
           {item.label}
         </Link>
       ))}
+      <Link
+        href="/settings"
+        className="ml-auto text-muted-foreground hover:text-foreground transition-colors"
+        aria-label="Settings"
+      >
+        <Settings className="size-4" />
+      </Link>
     </nav>
   );
 }