fix(VCheckboxBtn): reset checked state when readonly is true

[RafalOsieka]

Description

fixes #19137

How to reproduce the bug is described in the referenced issue, but shortly, the input internal checked state was out of sync with modelValue when the readonly flag is true.

That caused the bug, when the checkbox becomes interactive again, and sometimes it would not update the model value on the first click on the checkbox.

Markup:

<template>
  <v-app>
    <v-container>
      <v-checkbox-btn
        label="Checkbox #1:"
        v-model="checkboxState"
        :readonly="shouldLockCheckboxUpdates"
      />

      <v-checkbox-btn
        v-model="shouldLockCheckboxUpdates"
        label="Checkbox #2:"
      />
    </v-container>
  </v-app>
</template>

<script setup lang="ts">
import { ref } from 'vue'

const checkboxState = ref(false);
const shouldLockCheckboxUpdates = ref(false);
</script>