fix(VSelect): correct focus resolution in Shadow DOM

[lazerg]

Description

Inside a shadow root, document.activeElement only resolves to the shadow host, not the actual focused element, so onAfterEnter's containment check in VSelect always reports false and the fallback focus-move logic fires even when focus is already inside the list. This is the same issue #23024 and #23027 fixed for VDialog, VField, VTextField, and the rest; VSelect (and VAutocomplete/VCombobox by extension) was the one production usage they missed. Swapped document.activeElement for the existing getActiveElement() helper, the same one-line fix used in the sibling components.

Fixes #23101

Markup:

<template>
  <v-app>
    <v-container>
      <p class="mb-4">Open the select, then use arrow keys — focus should stay put instead of jumping back to the first item.</p>
      <shadow-demo />
    </v-container>
  </v-app>
</template>

<script setup lang="ts">
  import { defineComponent, defineCustomElement, h } from 'vue'
  import { VSelect } from '@/components/VSelect'
  import { VThemeProvider } from '@/components/VThemeProvider'
  import vuetify from './vuetify'

  const Demo = defineComponent(() => {
    return () => h(VSelect, {
      label: 'items',
      items: ['one', 'two', 'three', 'four', 'five'],
    })
  })

  const App = defineComponent({
    setup: () => () => h(VThemeProvider, { withBackground: true }, () => h(Demo)),
  })

  if (!customElements.get('vv-select')) {
    customElements.define('vv-select', defineCustomElement(App, {
      configureApp: app => app.use(vuetify),
    }))
  }

  const ShadowDemo = defineComponent({
    render: () => h('vv-select'),
  })
</script>