fix(mobile-next): 添加同事页工号提示与发送按钮改回独立 Obx

仅改 add_colleague_page.dart：directoryHint 与 sending 各用独立 Obx 追踪，去掉嵌套 Builder。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

mobile-next/lib/company/ui/add_colleague_page.dart

@@ -130,7 +130,7 @@ class AddColleaguePage extends StatelessWidget {
                   logic.lookupDirectory();
                 },
               ),
-              Builder(builder: (_) {
+              Obx(() {
                 final hint = logic.directoryHint.value;
                 if (hint == null) return const SizedBox.shrink();
                 return Padding(
@@ -153,7 +153,7 @@ class AddColleaguePage extends StatelessWidget {
                 enabled: !logic.sending.value,
               ),
               const Spacer(),
-              Builder(builder: (_) {
+              Obx(() {
                 final busy = logic.sending.value;
                 return SizedBox(
                   width: double.infinity,