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>

public/assets/css/style.css

@@ -234,8 +234,8 @@ img {
   --space-m: 2rem;
   --space-big: 3em;
   --curve: cubic-bezier(0.86, 0, 0.07, 1);
-  --sans-serif: "DM Sans", sans-serif;
-  --mono: "Inconsolata", monospace;
+  --sans-serif: 'DM Sans', sans-serif;
+  --mono: 'Inconsolata', monospace;
   --input-h: 26px;
   --input-w: 160px;
   --input-w-small: 45px;
@@ -688,4 +688,13 @@ input[type=number] {
   line-height: 1.5;
   resize: none;
   outline: none;
+}
+
+button[disabled] {
+  cursor: not-allowed !important;
+  opacity: 0.6;
+}
+
+button[disabled]:hover {
+  background-color: inherit !important;
 }/*# sourceMappingURL=style.css.map */