v0.2.0 - Design Editor: templates, patterns, header/footer variations

Full Design Editor compatibility with custom block categories, page templates,
header/footer variations, and navigation styles. Both FSE (admin) and Twig
(frontend) sides kept in sync.

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

src/scss/_custom.scss

@@ -25,6 +25,24 @@
     }
 }
 
+// Transparent header variant
+.header-transparent {
+    position: absolute;
+    width: 100%;
+    z-index: $zindex-fixed;
+
+    .navbar {
+        background: transparent !important;
+    }
+}
+
+// Offcanvas navigation dark mode compatibility
+[data-bs-theme="dark"] {
+    .offcanvas {
+        --bs-offcanvas-bg: var(--bs-body-bg);
+    }
+}
+
 // Dark mode overrides for block styles with hardcoded colors
 [data-bs-theme="dark"] {
     .is-style-alert-info {