fix: billing layout - wrap Form in fixed-height VBox, Tabular as filler

wwwroot/billing.ui

@@ -17,10 +17,16 @@
 			}
 		},
 		{
-			"widgettype":"Form",
+			"widgettype":"VBox",
 			"options":{
-				"height":"250px",
+				"width":"100%",
-				"fields":[
+				"height":"60px"
+			},
+			"subwidgets":[
+				{
+					"widgettype":"Form",
+					"options":{
+						"fields":[
 					{
 						"name":"start_date",
 						"label":"开始日期",
@@ -54,6 +60,8 @@
 					"mode":"replace"
 				}
 			]
+				}
+			]
 		}
 {% if start_date and end_date %}
 		,{