feat(analytics): typed Umami event taxonomy across the funnel

Add lib/analytics.ts with a single typed track() wrapper. SSR-safe,
never throws, no-ops when Umami isn't loaded. Event names form a
fixed union so refactors stay safe.

14 events wired:

  Discovery (3)
    search_submitted          FilterBar submit + near_me path
    near_me_used              all geolocation outcomes
    empty_results             search returns 0 schools

  Engagement (5)
    school_viewed             SchoolDetail + Secondary on mount, with
                              urn / phase / local_authority / from
    section_nav_used          section-nav links on both detail views
    chart_metric_changed      mobile chart chip switch
    metric_compared_in_rankings rankings metric dropdown
    external_link_clicked     Ofsted / school website / DfE (declarative
                              data-umami-event attributes)

  Conversion (5)
    compare_school_added      search/rankings/detail/compare sources
    compare_school_removed    detail toggle and compare page
    compare_viewed            once per session when there's a selection
                              (school_count, phase_mix)
    compare_metric_changed    compare page metric dropdown
    compare_shared            native sheet vs clipboard distinguished

  Operational (1)
    api_error                 caught in handleResponse, includes
                              endpoint / status / route

Suggested Goals to configure in the Umami dashboard for the funnel
report: search_submitted → school_viewed → compare_school_added →
compare_viewed → compare_shared.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/components/RankingsView.tsx

@@ -9,6 +9,7 @@ import { useRouter, usePathname, useSearchParams } from 'next/navigation';
 import { useComparison } from '@/hooks/useComparison';
 import type { RankingEntry, Filters, MetricDefinition } from '@/lib/types';
 import { formatPercentage, formatProgress, formatAcademicYear, schoolUrl } from '@/lib/utils';
+import { track } from '@/lib/analytics';
 import { EmptyState } from './EmptyState';
 import styles from './RankingsView.module.css';
 
@@ -79,6 +80,7 @@ export function RankingsView({
   };
 
   const handleMetricChange = (metric: string) => {
+    track('metric_compared_in_rankings', { metric, phase: selectedPhase });
     updateFilters({ metric });
   };
 
@@ -98,6 +100,7 @@ export function RankingsView({
       latitude: null,
       longitude: null,
     } as any);
+    track('compare_school_added', { urn: ranking.urn, from: 'rankings' });
   };
 
   // Get metric definition