style(agent_view): 优化输入框的边框样式和交互状态

添加聚焦状态边框样式并根据暗黑模式调整颜色，同时移除默认填充背景

lib/modules/agent/views/agent_view.dart

@@ -384,7 +384,16 @@ class AgentView extends StatelessWidget {
                                           color: Colors.grey[500],
                                           fontSize: 15,
                                         ),
+                                        filled: false,
                                         border: InputBorder.none,
+                                        focusedBorder: OutlineInputBorder(
+                                          borderSide: BorderSide(
+                                            color: isDarkMode
+                                                ? Colors.grey[800]!
+                                                : Colors.grey[200]!,
+                                            width: 1.0,
+                                          ),
+                                        ),
                                         contentPadding: EdgeInsets.symmetric(
                                             vertical: isImageActive ? 12 : 0),
                                       ),