Form: confirm dialog before submitting check-in

Adds a native window.confirm() inside onSubmit, after the uploads-in-flight
guard and before the submitting-state flip. Runs only after react-hook-form
validation passes, so users who hit Submit on an incomplete form still see
the existing field-level errors via onInvalid rather than a confusing
"are you sure?" prompt. Cancelling leaves the form state untouched.

components/EngagementForm.tsx

@@ -300,6 +300,12 @@ export function EngagementForm({ config, cid, cs }: EngagementFormProps) {
       });
       return;
     }
+    const confirmed = window.confirm(
+      "Submit your check-in?\n\n" +
+        "Please confirm your responses are complete and ready. " +
+        "After submitting, you can come back later and submit another check-in with updates.",
+    );
+    if (!confirmed) return;
     setSubmitState({ kind: "submitting" });
     try {
       // Strip values for hidden fields — never write data the user couldn't see.