Add new sections, update design system, fix header mobile layout

Sections:
- Add announcement-bar, cards, features-3col-cards, hero-card-homepage, hero-card-page, hero-slider
- Rename content-image-right → content-image (image-left layout, better mobile stacking)

Design system:
- Add cards-grid global class (flex-wrap for fluid wrapping card grids)
- Update feature-card with responsive _widthMax constraints (33%/50%/95%)
- Document Bricks Style Manager spacing setup in design.md

Headers:
- Fix header-with-cta mobile hamburger positioning — wrap nav+button in block element with row-reverse on mobile

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

design/bcw-global-classes.json

@@ -6,7 +6,7 @@
       "_typography": {
         "text-align": "center"
       },
-      "_rowGap": "10px"
+      "_rowGap": "var(--row-gap)"
     },
     "modified": 1772146285,
     "user_id": 2
@@ -15,7 +15,7 @@
     "id": "footer-sub-section",
     "name": "footer-sub-section",
     "settings": {
-      "_rowGap": "15px"
+      "_rowGap": "var(--row-gap)"
     },
     "modified": 1772146505,
     "user_id": 2
@@ -53,7 +53,7 @@
       "_alignItems": "flex-start",
       "_justifyContent": "center",
       "_gap": "32px",
-      "_rowGap:mobile_portrait": "15px"
+      "_rowGap:mobile_portrait": "var(--row-gap)"
     },
     "modified": 1772146505,
     "user_id": 2
@@ -66,9 +66,52 @@
       "_alignItems": "flex-start",
       "_justifyContent": "center",
       "_gap": "24px",
-      "_rowGap:mobile_portrait": "15px"
+      "_rowGap:mobile_portrait": "var(--row-gap)"
     },
     "modified": 1772146505,
     "user_id": 2
+  },
+  {
+    "id": "cards-grid",
+    "name": "cards-grid",
+    "settings": {
+      "_direction": "row",
+      "_flexWrap": "wrap",
+      "_alignItems": "stretch",
+      "_gap": "var(--col-gap)"
+    },
+    "modified": 1773187200,
+    "user_id": 2
+  },
+  {
+    "id": "feature-card",
+    "name": "feature-card",
+    "settings": {
+      "_background": {
+        "color": {
+          "raw": "var(--color-bg-alt)"
+        }
+      },
+      "_padding": {
+        "top": "var(--space-md)",
+        "right": "var(--space-md)",
+        "bottom": "var(--space-md)",
+        "left": "var(--space-md)"
+      },
+      "_border": {
+        "radius": {
+          "top": "8px",
+          "right": "8px",
+          "bottom": "8px",
+          "left": "8px"
+        }
+      },
+      "_rowGap": "var(--row-gap)",
+      "_widthMax": "33%",
+      "_widthMax:mobile_landscape": "50%",
+      "_widthMax:mobile_portrait": "95%"
+    },
+    "modified": 1773187200,
+    "user_id": 2
   }
 ]