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/app/page.tsx

@@ -3,7 +3,7 @@
  * Main landing page with school search and browsing
  */
 
-import { fetchSchools, fetchFilters } from '@/lib/api';
+import { fetchSchools, fetchFilters, fetchDataInfo } from '@/lib/api';
 import { HomeView } from '@/components/HomeView';
 
 interface HomePageProps {
@@ -43,7 +43,7 @@ export default async function HomePage({ searchParams }: HomePageProps) {
 
   // Fetch data on server with error handling
   try {
-    const filtersData = await fetchFilters();
+    const [filtersData, dataInfo] = await Promise.all([fetchFilters(), fetchDataInfo().catch(() => null)]);
 
     // Only fetch schools if there are search parameters
     let schoolsData;
@@ -66,6 +66,7 @@ export default async function HomePage({ searchParams }: HomePageProps) {
       <HomeView
         initialSchools={schoolsData}
         filters={filtersData || { local_authorities: [], school_types: [], years: [] }}
+        totalSchools={dataInfo?.total_schools ?? null}
       />
     );
   } catch (error) {
@@ -76,6 +77,7 @@ export default async function HomePage({ searchParams }: HomePageProps) {
       <HomeView
         initialSchools={{ schools: [], page: 1, page_size: 50, total: 0, total_pages: 0 }}
         filters={{ local_authorities: [], school_types: [], years: [] }}
+        totalSchools={null}
       />
     );
   }