fix: migrate DOM widgets to ComponentWidgetImpl and add element value bridge

- Add useDomValueBridge composable that bridges DOM element .value to Vue
  reactivity via Object.defineProperty interception + input events
- Wire bridge into addDOMWidget for extension widget compatibility
- Migrate useStringWidget from raw textarea + addDOMWidget to
  ComponentWidgetImpl + WidgetTextarea.vue
- Migrate useMarkdownWidget from Tiptap + addDOMWidget to
  ComponentWidgetImpl + WidgetMarkdown.vue
- Remove all Tiptap dependencies (no longer used anywhere)
- Add spellcheck setting support to WidgetTextarea.vue

Fixes #9194

Amp-Thread-ID: https://ampcode.com/threads/T-019c977f-02f8-701d-b258-95157da8c261

src/renderer/extensions/vueNodes/widgets/components/WidgetTextarea.test.ts

@@ -1,3 +1,4 @@
+import { createTestingPinia } from '@pinia/testing'
 import { mount } from '@vue/test-utils'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
@@ -39,6 +40,9 @@ function mountComponent(
   placeholder?: string
 ) {
   return mount(WidgetTextarea, {
+    global: {
+      plugins: [createTestingPinia()]
+    },
     props: {
       widget,
       modelValue,