fix: race condition on custom styles loading + misc cleanup

Guard watcher-triggered renders in usePreviewRenderer until first
explicit render completes, preventing premature renders with default
styles. Also: disable Contenu tab, update content/blueprints, add
global disabled button styles, minor formatting.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/App.vue

@@ -61,10 +61,7 @@ const activeFrame = computed(() => {
 const { printPreview } = usePrintPreview(activeFrame);
 
 // Setup keyboard shortcuts (depends on printPreview)
-const {
-  handleKeyboardShortcut,
-  isMac
-} = useKeyboardShortcuts({
+const { handleKeyboardShortcut, isMac } = useKeyboardShortcuts({
   stylesheetStore,
   elementPopup,
   // pagePopup, // DISABLED: page template styling feature
@@ -125,7 +122,11 @@ onMounted(async () => {
   />
   -->
 
-  <button class="print-btn" @click="printPreview" :title="`Imprimer (${isMac ? '⌘' : 'Ctrl'}+P)`">
+  <button
+    class="print-btn"
+    @click="printPreview"
+    :title="`Imprimer (${isMac ? '⌘' : 'Ctrl'}+P)`"
+  >
     <svg
       xmlns="http://www.w3.org/2000/svg"
       viewBox="0 0 24 24"