feat(ui): site-wide UX/UI audit — unified buttons, tokens, accessibility

- Add shared button system (.btn-primary/secondary/tertiary/active) in globals.css
- Replace 40+ hardcoded rgba() values with design tokens across all CSS modules
- Add skip link, :focus-visible indicators, and ARIA landmarks
- Standardise button labels ("+ Compare" / "✓ Comparing") across all components
- Add collapse/minimize toggle to ComparisonToast
- Fix heading hierarchy (h3→h2 in ComparisonView)
- Add aria-live on search results, aria-label on trend SVGs
- Add "Search" nav link, fix footer empty section, unify max-widths
- Darken --text-muted for WCAG AA compliance (4.6:1 contrast ratio)
- Net reduction of ~180 lines through button style deduplication

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

nextjs-app/components/Navigation.module.css

@@ -8,7 +8,7 @@
 }
 
 .container {
-  max-width: 1280px;
+  max-width: 1400px;
   margin: 0 auto;
   padding: 0 1.5rem;
   display: flex;
@@ -89,7 +89,7 @@
 
 .navLink.active {
   color: var(--accent-coral, #e07256);
-  background: rgba(224, 114, 86, 0.1);
+  background: var(--accent-coral-bg);
 }
 
 .navLink.active::after {