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/ComparisonToast.module.css

@@ -64,20 +64,6 @@
   border-top: 1px solid rgba(255, 255, 255, 0.1);
 }
 
-.btnClear {
-  background: transparent;
-  border: none;
-  color: rgba(250, 247, 242, 0.7);
-  font-size: 0.85rem;
-  font-weight: 500;
-  cursor: pointer;
-  padding: 0.5rem;
-  transition: color 0.2s ease;
-}
-
-.btnClear:hover {
-  color: var(--text-inverse, #faf7f2);
-}
 
 .btnCompare {
   background: white;
@@ -99,9 +85,31 @@
 .toastHeader {
   display: flex;
   align-items: center;
+  justify-content: space-between;
   margin-bottom: 0.5rem;
 }
 
+.toastCollapsed .toastHeader {
+  margin-bottom: 0;
+}
+
+.collapseBtn {
+  background: none;
+  border: none;
+  color: rgba(250, 247, 242, 0.6);
+  cursor: pointer;
+  padding: 0.25rem;
+  line-height: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  transition: color 0.2s ease;
+}
+
+.collapseBtn:hover {
+  color: var(--text-inverse, #faf7f2);
+}
+
 .toastTitle {
   display: flex;
   align-items: center;