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/SchoolSearchModal.tsx

@@ -10,6 +10,7 @@ import { Modal } from "./Modal";
 import { useComparison } from "@/hooks/useComparison";
 import { debounce } from "@/lib/utils";
 import { fetchSchools } from "@/lib/api";
+import { track } from "@/lib/analytics";
 import type { School } from "@/lib/types";
 import styles from "./SchoolSearchModal.module.css";
 
@@ -60,6 +61,11 @@ export function SchoolSearchModal({ isOpen, onClose }: SchoolSearchModalProps) {
 
   const handleAddSchool = (school: School) => {
     addSchool(school);
+    track('compare_school_added', {
+      urn: school.urn,
+      from: 'compare',
+      selection_count_after: selectedSchools.length + 1,
+    });
     // Don't close modal, allow adding multiple schools
   };