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-css-variables.json

@@ -134,64 +134,76 @@
       "value": "96px",
       "category": "bcwspc"
     },
+    {
+      "id": "bcwv31",
+      "name": "row-gap",
+      "value": "15px",
+      "category": "bcwspc"
+    },
+    {
+      "id": "bcwv32",
+      "name": "col-gap",
+      "value": "15px",
+      "category": "bcwspc"
+    },
     {
       "id": "bcwv21",
       "name": "h1",
-      "value": "3rem",
+      "value": "64px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv22",
       "name": "h2",
-      "value": "2.25rem",
+      "value": "48px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv23",
       "name": "h3",
-      "value": "1.75rem",
+      "value": "36px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv24",
       "name": "h4",
-      "value": "1.25rem",
+      "value": "28px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv25",
       "name": "h5",
-      "value": "1.125rem",
+      "value": "22px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv26",
       "name": "h6",
-      "value": "1rem",
+      "value": "18px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv27",
       "name": "text-sm",
-      "value": "0.875rem",
+      "value": "14px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv28",
       "name": "text-base",
-      "value": "1rem",
+      "value": "16px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv29",
       "name": "text-lg",
-      "value": "1.125rem",
+      "value": "18px",
       "category": "bcwtyp"
     },
     {
       "id": "bcwv30",
       "name": "text-xl",
-      "value": "1.25rem",
+      "value": "20px",
       "category": "bcwtyp"
     }
   ]