fix(VFileInput): show drag feedback for file drops

[mahsarajabpour]

Description

Fixes #20576.

This improves drag-and-drop feedback in v-file-input.

The dragging state is now properly cleared when a file drag leaves the input, preventing the state from becoming stuck.

The previous feedback effect produced inconsistent results across variants (filled, solo-filled, and solo-inverted) and depended on the primary theme color. It has been replaced with a simple dashed outline that provides consistent drag feedback across variants and themes.

Tests were updated to cover drag state cleanup and drag feedback behavior.

Markup

<template>
  <v-sheet class="mb-8 pa-6" border rounded>
    <h2 class="text-h6 mb-4">
      VFileInput drag feedback test
    </h2>

    <v-row>
      <v-col v-for="variant in ['outlined', 'filled', 'solo', 'solo-inverted', 'solo-filled']" :key="variant" cols="12"
             md="6">
        <v-file-input
          :label="variant"
          :variant="variant"
          multiple
        />
      </v-col>
    </v-row>
  </v-sheet>
</template>
<script setup lang="ts">
</script>

[J-Sek]

• when dragging, leaving to the side - the dragging state is stuck
• the effect could is almost not visible for the filled variant, solo-inverted flashes solid color, solo-filled does not respond at all
• we cannot use primary theme color here

I wonder if replacing the effect with simple dashed outline would help work around some of the problems.

BTW. PR needs some playground so we can easily paste it locally to dev/Playground.vue for verification.