feat(phase-18): plan 02 — consolidate analytics into admin dashboard

- Merge /admin/analytics content (MetricCard, MonthlyChart, time tracking) into /admin
- AdminDashboard now accepts searchParams and year query param
- Add getAnalyticsByYear, getMonthlyCollected, getAvailableYears, getTimeByClient, getTotalTrackedHours queries
- Unify fmtEur to accept number (more robust); update KPI card callers with parseFloat()
- Add MetricCard and fmtSeconds helpers to admin/page.tsx
- YearSelector navigates to /admin?year=Y (was /admin/analytics?year=Y)
- Delete src/app/admin/analytics/page.tsx — /admin/analytics now returns 404

src/components/admin/YearSelector.tsx

@@ -15,7 +15,7 @@ export function YearSelector({
   const thisYear = new Date().getFullYear();
 
   function go(y: number) {
-    router.push(`/admin/analytics?year=${y}`);
+    router.push(`/admin?year=${y}`);
   }
 
   return (