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/ComparisonView.module.css

@@ -30,24 +30,6 @@
   line-height: 1.6;
 }
 
-.addButton {
-  padding: 0.625rem 1.25rem;
-  font-size: 0.9rem;
-  font-weight: 600;
-  background: var(--accent-coral, #e07256);
-  color: white;
-  border: none;
-  border-radius: 8px;
-  cursor: pointer;
-  transition: all 0.2s ease;
-  white-space: nowrap;
-}
-
-.addButton:hover {
-  background: var(--accent-coral-dark, #c45a3f);
-  transform: translateY(-1px);
-  box-shadow: 0 4px 12px rgba(224, 114, 86, 0.3);
-}
 
 /* Metric Selector */
 .metricSelector {
@@ -90,7 +72,7 @@
 .metricSelect:focus {
   outline: none;
   border-color: var(--accent-coral, #e07256);
-  box-shadow: 0 0 0 3px rgba(224, 114, 86, 0.15);
+  box-shadow: 0 0 0 3px var(--accent-coral-bg);
 }
 
 .metricSelect optgroup {
@@ -366,25 +348,6 @@
   border-radius: 0 var(--radius-sm) var(--radius-sm) 0;
 }
 
-.shareButton {
-  display: inline-flex;
-  align-items: center;
-  gap: 0.375rem;
-  padding: 0.625rem 1.25rem;
-  background: var(--bg-secondary);
-  border: 1px solid var(--border-color, #e0ddd8);
-  border-radius: 8px;
-  color: var(--text-secondary);
-  font-size: 0.9rem;
-  font-weight: 600;
-  cursor: pointer;
-  transition: all var(--transition);
-}
-
-.shareButton:hover {
-  background: var(--bg-primary);
-  color: var(--text-primary);
-}
 
 /* Responsive Design */
 @media (max-width: 768px) {
@@ -393,10 +356,6 @@
     align-items: stretch;
   }
 
-  .addButton {
-    width: 100%;
-  }
-
   .header h1 {
     font-size: 1.75rem;
   }