feat: add books/pages metric toggle on reading activity chart

Allow switching between number of books and number of pages on the
dashboard reading activity chart. Adds pages_read to the stats API
response and a MetricToggle component alongside the existing PeriodToggle.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/backoffice/lib/i18n/fr.ts

@@ -78,6 +78,8 @@ const fr = {
   "dashboard.periodDay": "Jour",
   "dashboard.periodWeek": "Semaine",
   "dashboard.periodMonth": "Mois",
+  "dashboard.metricBooks": "Livres",
+  "dashboard.metricPages": "Pages",
   "dashboard.popularSeries": "Séries populaires",
   "dashboard.noSeries": "Aucune série pour le moment",
   "dashboard.unknown": "Inconnu",