feat: add CSS file import with drag & drop support

Add CssFileImport component to StylesheetViewer allowing users to
import CSS files to replace custom CSS content.

Features:
- Click to select file via file dialog
- Drag & drop support with visual feedback
- File validation (.css only, max 1MB)
- Error messages for invalid files
- Direct replacement of customCss content

New component:
- src/components/ui/CssFileImport.vue

Integration:
- Added at top of StylesheetViewer
- Emits 'import' event with file content
- Content replaces customCss in store

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/components/StylesheetViewer.vue

@@ -1,5 +1,8 @@
 <template>
   <div id="stylesheet-viewer">
+    <!-- CSS File Import -->
+    <CssFileImport @import="handleCssImport" />
+
     <!-- Base CSS Section (Collapsable, closed by default) -->
     <div class="css-section">
       <div class="section-header" @click="isBaseCssExpanded = !isBaseCssExpanded">
@@ -53,6 +56,7 @@
 <script setup>
 import { ref, computed, watch } from 'vue';
 import { useStylesheetStore } from '../stores/stylesheet';
+import CssFileImport from './ui/CssFileImport.vue';
 import hljs from 'highlight.js/lib/core';
 import css from 'highlight.js/lib/languages/css';
 import 'highlight.js/styles/atom-one-dark.css';
@@ -90,6 +94,11 @@ const handleFocus = () => {
   stylesheetStore.isEditing = true;
 };
 
+const handleCssImport = (cssContent) => {
+  // Replace custom CSS with imported content
+  stylesheetStore.customCss = cssContent;
+};
+
 // Watch editing mode and format when exiting
 watch(isCustomCssEditable, async (newValue, oldValue) => {
   stylesheetStore.isEditing = newValue;

src/components/ui/CssFileImport.vue

@@ -0,0 +1,159 @@
+<template>
+  <div
+    class="css-import"
+    :class="{ 'is-dragging': isDragging }"
+    @dragover.prevent="handleDragOver"
+    @dragleave.prevent="handleDragLeave"
+    @drop.prevent="handleDrop"
+  >
+    <input
+      ref="fileInput"
+      type="file"
+      accept=".css"
+      @change="handleFileSelect"
+      style="display: none"
+    />
+
+    <button class="import-button" @click="openFileDialog" type="button">
+      <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="currentColor">
+        <path d="M9 16h6v-6h4l-7-7-7 7h4zm-4 2h14v2H5z"/>
+      </svg>
+      <span>Importer un fichier CSS</span>
+    </button>
+
+    <span class="import-hint">ou glisser-déposer ici</span>
+
+    <div v-if="error" class="import-error">{{ error }}</div>
+  </div>
+</template>
+
+<script setup>
+import { ref } from 'vue';
+
+const emit = defineEmits(['import']);
+
+const fileInput = ref(null);
+const isDragging = ref(false);
+const error = ref('');
+
+const openFileDialog = () => {
+  fileInput.value?.click();
+};
+
+const handleDragOver = (e) => {
+  isDragging.value = true;
+};
+
+const handleDragLeave = (e) => {
+  isDragging.value = false;
+};
+
+const handleDrop = (e) => {
+  isDragging.value = false;
+  const files = e.dataTransfer.files;
+
+  if (files.length > 0) {
+    processFile(files[0]);
+  }
+};
+
+const handleFileSelect = (e) => {
+  const files = e.target.files;
+
+  if (files.length > 0) {
+    processFile(files[0]);
+  }
+};
+
+const processFile = (file) => {
+  error.value = '';
+
+  // Validate file type
+  if (!file.name.endsWith('.css')) {
+    error.value = 'Seuls les fichiers .css sont acceptés';
+    return;
+  }
+
+  // Validate file size (max 1MB)
+  if (file.size > 1024 * 1024) {
+    error.value = 'Le fichier est trop volumineux (max 1MB)';
+    return;
+  }
+
+  // Read file content
+  const reader = new FileReader();
+
+  reader.onload = (e) => {
+    const content = e.target.result;
+    emit('import', content);
+
+    // Reset input
+    if (fileInput.value) {
+      fileInput.value.value = '';
+    }
+  };
+
+  reader.onerror = () => {
+    error.value = 'Erreur lors de la lecture du fichier';
+  };
+
+  reader.readAsText(file);
+};
+</script>
+
+<style scoped>
+.css-import {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  padding: 0.75rem 1rem;
+  background: #21252b;
+  border: 2px dashed transparent;
+  border-radius: 0.25rem;
+  transition: all 0.2s ease;
+}
+
+.css-import.is-dragging {
+  border-color: #61afef;
+  background: #2c313c;
+}
+
+.import-button {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  padding: 0.5rem 0.75rem;
+  background: #2c313c;
+  color: #abb2bf;
+  border: 1px solid #3e4451;
+  border-radius: 0.25rem;
+  font-size: 0.875rem;
+  cursor: pointer;
+  transition: all 0.2s ease;
+}
+
+.import-button:hover {
+  background: #3e4451;
+  border-color: #61afef;
+  color: #61afef;
+}
+
+.import-button svg {
+  width: 1rem;
+  height: 1rem;
+}
+
+.import-hint {
+  font-size: 0.75rem;
+  color: #5c6370;
+  font-style: italic;
+}
+
+.import-error {
+  font-size: 0.75rem;
+  color: #e06c75;
+  background: rgba(224, 108, 117, 0.1);
+  padding: 0.25rem 0.5rem;
+  border-radius: 0.25rem;
+}
+</style>