feat: add fixed sidebar navigation with hover expand and mobile hamburger

Implements Task 3: fixed left sidebar (70px collapsed → 250px on hover) with
9 nav items, social links, active-state detection, and mobile hamburger toggle
that matches the existing sidebar-nav.js selectors.

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

wp-content/themes/bitwms-child/functions.php

@@ -52,3 +52,11 @@ foreach ( $bitwms_includes as $bitwms_file ) {
     }
 }
 unset( $bitwms_file, $bitwms_path );
+
+// ---------------------------------------------------------------------------
+// 3. Inject sidebar navigation before the GeneratePress header
+// ---------------------------------------------------------------------------
+
+add_action( 'generate_before_header', function () {
+    get_template_part( 'templates/sidebar-nav' );
+} );