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/SchoolDetailView.tsx

@@ -23,6 +23,7 @@ import {
 } from '@/lib/utils';
 import { DeltaChip } from './DeltaChip';
 import SatsChart from './SatsChart';
+import { track, getNavigationSource } from '@/lib/analytics';
 import styles from './SchoolDetailView.module.css';
 
 const OFSTED_LABELS: Record<number, string> = {
@@ -121,11 +122,24 @@ export function SchoolDetailView({
   const handleComparisonToggle = () => {
     if (isInComparison) {
       removeSchool(schoolInfo.urn);
+      track('compare_school_removed', { urn: schoolInfo.urn, from: 'detail' });
     } else {
       addSchool(schoolInfo);
+      track('compare_school_added', { urn: schoolInfo.urn, from: 'detail' });
     }
   };
 
+  // Page-view event with funnel attribution. Fires once per mount.
+  useEffect(() => {
+    track('school_viewed', {
+      urn: schoolInfo.urn,
+      phase: phase || 'unknown',
+      local_authority: schoolInfo.local_authority || 'unknown',
+      from: getNavigationSource(),
+    });
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [schoolInfo.urn]);
+
   const deprivationDesc = (decile: number) => {
     if (decile <= 3) return `This school is in one of England's most deprived areas (decile ${decile}/10). Many pupils may face additional challenges at home.`;
     if (decile <= 7) return `This school is in an area with average levels of deprivation (decile ${decile}/10).`;
@@ -261,7 +275,13 @@ export function SchoolDetailView({
               )}
               {schoolInfo.website && (
                 <span className={styles.headerDetail}>
-                  <a href={/^https?:\/\//i.test(schoolInfo.website) ? schoolInfo.website : `https://${schoolInfo.website}`} target="_blank" rel="noopener noreferrer">
+                  <a
+                    href={/^https?:\/\//i.test(schoolInfo.website) ? schoolInfo.website : `https://${schoolInfo.website}`}
+                    target="_blank"
+                    rel="noopener noreferrer"
+                    data-umami-event="external_link_clicked"
+                    data-umami-event-target="school_website"
+                  >
                     School website ↗
                   </a>
                 </span>
@@ -395,6 +415,7 @@ export function SchoolDetailView({
               key={id}
               href={`#${id}`}
               className={`${styles.sectionNavLink}${activeSection === id ? ` ${styles.sectionNavLinkActive}` : ''}`}
+              onClick={() => track('section_nav_used', { section: id })}
             >
               {label}
             </a>
@@ -417,6 +438,8 @@ export function SchoolDetailView({
               target="_blank"
               rel="noopener noreferrer"
               className={styles.ofstedReportLink}
+              data-umami-event="external_link_clicked"
+              data-umami-event-target="ofsted"
             >
               Full report ↗
             </a>