refactor(frontend): Formまわりの型強化 (#16260)

* refactor(frontend): Formまわりの型強化

* fix

* avoid non-null assertion and add null check for safety

* refactor

* avoid non-null assertion and add null check for safety

* Update clip.vue

---------

Co-authored-by: syuilo <4439005+syuilo@users.noreply.github.com>

packages/frontend/src/widgets/WidgetAichan.vue

@@ -13,16 +13,16 @@ SPDX-License-Identifier: AGPL-3.0-only
 import { onMounted, onUnmounted, useTemplateRef } from 'vue';
 import { useWidgetPropsManager } from './widget.js';
 import type { WidgetComponentProps, WidgetComponentEmits, WidgetComponentExpose } from './widget.js';
-import type { GetFormResultType } from '@/utility/form.js';
+import type { FormWithDefault, GetFormResultType } from '@/utility/form.js';
 
 const name = 'ai';
 
 const widgetPropsDef = {
 	transparent: {
-		type: 'boolean' as const,
+		type: 'boolean',
 		default: false,
 	},
-};
+} satisfies FormWithDefault;
 
 type WidgetProps = GetFormResultType<typeof widgetPropsDef>;
 
@@ -42,6 +42,8 @@ const touched = () => {
 };
 
 const onMousemove = (ev: MouseEvent) => {
+	if (!live2d.value || !live2d.value.contentWindow) return;
+
 	const iframeRect = live2d.value.getBoundingClientRect();
 	live2d.value.contentWindow.postMessage({
 		type: 'moveCursor',