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/features-3col.json

@@ -10,7 +10,7 @@
         "_typography": {
           "text-align": "center"
         },
-        "_rowGap": "10px"
+        "_rowGap": "var(--row-gap)"
       },
       "modified": 1772146285,
       "user_id": 2
@@ -73,7 +73,7 @@
         "_alignItems": "flex-start",
         "_justifyContent": "center",
         "_gap": "32px",
-        "_rowGap:mobile_portrait": "15px"
+        "_rowGap:mobile_portrait": "var(--row-gap)"
       },
       "label": "Features Row"
     },