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>

sections/hero-card-page.json

@@ -0,0 +1,79 @@
+{
+  "title": "BCW — Hero Card (Page)",
+  "type": "section",
+  "templateType": "section",
+  "content": [
+    {
+      "id": "sec001",
+      "name": "section",
+      "parent": 0,
+      "children": ["con001"],
+      "settings": {
+        "_background": {
+          "image": {
+            "url": "https://picsum.photos/1400/700",
+            "external": true,
+            "filename": "700"
+          }
+        },
+        "_padding": {
+          "top": "var(--space-xl)",
+          "bottom": "var(--space-xl)"
+        }
+      },
+      "label": "Hero Card (Page)"
+    },
+    {
+      "id": "con001",
+      "name": "container",
+      "parent": "sec001",
+      "children": ["con002"],
+      "settings": {
+        "_direction": "row",
+        "_alignItems": "center",
+        "_justifyContent": "center",
+        "_width": "100%",
+        "_maxWidth": "100%"
+      }
+    },
+    {
+      "id": "con002",
+      "name": "container",
+      "parent": "con001",
+      "children": ["hdg001"],
+      "settings": {
+        "_direction": "row",
+        "_alignItems": "flex-start",
+        "_justifyContent": "center",
+        "_gap": "var(--space-md)",
+        "_width": "50%",
+        "_width:mobile_portrait": "100%",
+        "_background": {
+          "color": {
+            "raw": "rgba(255, 255, 255, 0.88)"
+          }
+        },
+        "_padding": {
+          "top": "var(--space-lg)",
+          "right": "var(--space-lg)",
+          "bottom": "var(--space-lg)",
+          "left": "var(--space-lg)"
+        }
+      },
+      "label": "Content Card"
+    },
+    {
+      "id": "hdg001",
+      "name": "heading",
+      "parent": "con002",
+      "children": [],
+      "settings": {
+        "tag": "h1",
+        "text": "Page Title Goes Here",
+        "_typography": {
+          "text-align": "center"
+        }
+      }
+    }
+  ]
+}