feat: add scrollable CSS editor and complete stylesheet export

Add two improvements to StylesheetViewer:

1. Scrollable CSS sections
   - Add max-height (500px) and overflow-y to custom CSS editor
   - Applies to both read-only and editable modes
   - Improves UX when content exceeds viewport

2. Complete stylesheet export button
   - Exports merged base CSS + custom CSS to single file
   - Filename format: <narrative-slug>-style.print.css
   - Includes informative comments:
     * Header with narrative title and download date
     * Section markers for base CSS and custom CSS
   - Full-width button below custom CSS section
   - Download via blob + automatic cleanup

Export file structure:
- Header comment (narrative info, date)
- Base CSS section with comment
- Custom CSS section with comment

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

src/components/StylesheetViewer.vue

@@ -50,12 +50,21 @@
         ></textarea>
       </div>
     </div>
+
+    <!-- Export Button -->
+    <button class="export-button" @click="handleExport" type="button">
+      <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="currentColor">
+        <path d="M9 16v-6H5l7-7 7 7h-4v6H9zm-4 4h14v-2H5v2z"/>
+      </svg>
+      <span>Exporter la feuille de style complète</span>
+    </button>
   </div>
 </template>
 
 <script setup>
 import { ref, computed, watch } from 'vue';
 import { useStylesheetStore } from '../stores/stylesheet';
+import { useNarrativeStore } from '../stores/narrative';
 import CssFileImport from './ui/CssFileImport.vue';
 import hljs from 'highlight.js/lib/core';
 import css from 'highlight.js/lib/languages/css';
@@ -64,6 +73,7 @@ import 'highlight.js/styles/atom-one-dark.css';
 hljs.registerLanguage('css', css);
 
 const stylesheetStore = useStylesheetStore();
+const narrativeStore = useNarrativeStore();
 const isBaseCssExpanded = ref(false);
 const isCustomCssEditable = ref(false);
 let debounceTimer = null;
@@ -99,6 +109,65 @@ const handleCssImport = (cssContent) => {
   stylesheetStore.customCss = cssContent;
 };
 
+const handleExport = () => {
+  const now = new Date();
+  const dateStr = now.toLocaleDateString('fr-FR', {
+    day: '2-digit',
+    month: '2-digit',
+    year: 'numeric',
+    hour: '2-digit',
+    minute: '2-digit'
+  });
+
+  const narrativeTitle = narrativeStore.data?.title || 'Sans titre';
+  const narrativeSlug = narrativeStore.data?.slug || 'narrative';
+
+  // Build complete CSS with comments
+  let completeCSS = `/*
+ * Feuille de style pour l'impression
+ * Récit : ${narrativeTitle}
+ * Téléchargé le : ${dateStr}
+ *
+ * Ce fichier contient le CSS de base et le CSS personnalisé
+ * fusionnés pour une utilisation hors ligne.
+ */
+
+`;
+
+  // Add base CSS
+  if (stylesheetStore.baseCss) {
+    completeCSS += `/* ========================================
+ * CSS DE BASE
+ * Styles par défaut de l'application
+ * ======================================== */
+
+${stylesheetStore.baseCss}
+
+`;
+  }
+
+  // Add custom CSS
+  if (stylesheetStore.customCss) {
+    completeCSS += `/* ========================================
+ * CSS PERSONNALISÉ
+ * Styles spécifiques à ce récit
+ * ======================================== */
+
+${stylesheetStore.customCss}`;
+  }
+
+  // Create blob and download
+  const blob = new Blob([completeCSS], { type: 'text/css' });
+  const url = URL.createObjectURL(blob);
+  const link = document.createElement('a');
+  link.href = url;
+  link.download = `${narrativeSlug}-style.print.css`;
+  document.body.appendChild(link);
+  link.click();
+  document.body.removeChild(link);
+  URL.revokeObjectURL(url);
+};
+
 // Watch editing mode and format when exiting
 watch(isCustomCssEditable, async (newValue, oldValue) => {
   stylesheetStore.isEditing = newValue;
@@ -234,6 +303,7 @@ h3 {
   font-family: 'Courier New', Courier, monospace;
   font-size: 0.875rem;
   line-height: 1.5;
+  max-height: 500px;
 }
 
 .readonly code {
@@ -244,6 +314,8 @@ textarea {
   width: 100%;
   flex: 1;
   min-height: 300px;
+  max-height: 500px;
+  overflow-y: auto;
   background: #1e1e1e;
   color: #abb2bf;
   border: none;
@@ -259,4 +331,33 @@ textarea::placeholder {
   color: #5c6370;
   font-style: italic;
 }
+
+.export-button {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 0.5rem;
+  width: 100%;
+  padding: 0.75rem 1rem;
+  margin-top: 1rem;
+  background: #2c313c;
+  color: #abb2bf;
+  border: 1px solid #3e4451;
+  border-radius: 0.25rem;
+  font-size: 0.875rem;
+  font-weight: 500;
+  cursor: pointer;
+  transition: all 0.2s ease;
+}
+
+.export-button:hover {
+  background: #3e4451;
+  border-color: #61afef;
+  color: #61afef;
+}
+
+.export-button svg {
+  width: 1.25rem;
+  height: 1.25rem;
+}
 </style>