fix(useDisplay): more intuitive use of mobile-breakpoint

[J-Sek]

resolves #19883

Description

The mobile-breakpoint prop requires :mobile='null' to work, otherwise it is ignored. It is not intuitive and developers ask about it on Discord or assume it does not work, because components' API docs don't mention this requirement.

The reason Docs do not use description from api-generator » ... » display.json is that most components already have their own description for mobile prop.

I wish we could let user work with mobile-breakpoint and don't bother about the other prop. The logic could then be:

1. If mobile === true, then force mobile view
2. If mobile-breakpoint is set, accept it's value to resolve mobile state
3. All defaults (not using these props) means no responsiveness (as suggested by comment under [Bug Report][3.6.1] VDataTable mobile mode is based on screen width, not container width #19726)
4. mobile='null' makes the component rely on default mobile breakpoint

Markup:

<template>
  <v-app theme="dark">
    <v-container>
      <pre class="mt-8">
      $vuetify.display.width: {{ $vuetify.display.width }}px
      mobile-breakpoint="md" instructs labels to hide below the 960px (md breakpoint)
      </pre>
      <v-stepper mobile-breakpoint="md">
        <v-stepper-header>
          <template v-for="(item, i) in items" :key="i">
            <v-divider v-if="i" />
            <v-stepper-item v-bind="item" />
          </template>
        </v-stepper-header>
      </v-stepper>

      <pre class="mt-8">
      $vuetify.display.width: {{ $vuetify.display.width }}px
      :mobile-breakpoint="800" instructs labels to hide below the 800px
      </pre>
      <v-stepper :mobile-breakpoint="800">
        <v-stepper-header>
          <template v-for="(item, i) in items" :key="i">
            <v-divider v-if="i" />
            <v-stepper-item v-bind="item" />
          </template>
        </v-stepper-header>
      </v-stepper>
    </v-container>
  </v-app>
</template>

<script setup lang="ts">
  const items = Array.from({ length: 4 }).map((_, i) => ({
    title: `Step ${i + 1}`,
    subtitle: `Step ${i + 1} subtitle`,
    value: i + 1,
  }))
</script>