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>

.gitignore

@@ -1,3 +1,3 @@
 Updated_Sections/
 .claude/settings.local.json
-*.tmp.*
+*.tmp.*