feat(ux): implement comprehensive UX audit fixes across all pages

Addresses 28 issues identified in UX audit (P0–P3 severity):

P0 — Critical:
- Fix compare URL sharing: seed ComparisonContext from SSR initialData
  when localStorage is empty, making /compare?urns=... links shareable
- Remove permanently broken "Avg. Scaled Score" column from school
  detail historical data table

P1 — High priority:
- Add radius selector (0.5–10 mi) to postcode search in FilterBar
- Make Add to Compare a toggle (remove) on SchoolCards
- Hide hero title/description once a search is active
- Show school count + quick-search prompts on empty landing page
- Compare empty state opens in-page school search modal directly
- Remove URN from school detail header (irrelevant to end users)
- Move map above performance chart in school detail page
- Add ← Back navigation to school detail page
- Add sort controls to search results (RWM%, distance, A–Z)
- Show metric descriptions below metric selector
- Expand ComparisonToast to list school names with per-school remove
- Add progress score explainer (0 = national average) throughout

P2 — Medium:
- Remove console.log statements from ComparisonView
- Colour-code comparison school cards to match chart line colours
- Replace plain loading text with LoadingSkeleton in ComparisonView
- Rankings empty state uses shared EmptyState component
- Rankings year filter shows actual year e.g. "2023 (Latest)"
- Rankings subtitle shows top-N count
- Add View link alongside Add button in rankings table
- Remove placeholder Privacy Policy / Terms links from footer
- Replace untappable 10px info icons with visible metric hint text
- Show active filter chips in search results header

P3 — Polish:
- Remove redundant "Home" nav link (logo already links home)
- Add / and Ctrl+K keyboard shortcut to focus search input
- Add Share button to compare page (copies URL to clipboard)

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

nextjs-app/components/RankingsView.module.css

@@ -350,3 +350,39 @@
     min-width: 100px;
   }
 }
+
+.limitNote {
+  color: var(--text-muted);
+  font-weight: 400;
+}
+
+.metricDescription {
+  font-size: 0.875rem;
+  color: var(--text-secondary);
+  margin: -1rem 0 1.5rem;
+  max-width: 700px;
+}
+
+.progressHint {
+  font-size: 0.8rem;
+  color: var(--text-muted);
+  margin: -1rem 0 1.5rem;
+  font-style: italic;
+}
+
+.viewButton {
+  display: inline-block;
+  padding: 0.25rem 0.625rem;
+  font-size: 0.8rem;
+  color: var(--text-secondary);
+  border: 1px solid var(--border-color, #e0ddd8);
+  border-radius: var(--radius-sm, 4px);
+  text-decoration: none;
+  margin-right: 0.375rem;
+  transition: all var(--transition, 0.2s ease);
+}
+
+.viewButton:hover {
+  background: var(--bg-secondary);
+  color: var(--text-primary);
+}