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/en.ts

@@ -80,6 +80,8 @@ const en: Record<TranslationKey, string> = {
   "dashboard.periodDay": "Day",
   "dashboard.periodWeek": "Week",
   "dashboard.periodMonth": "Month",
+  "dashboard.metricBooks": "Books",
+  "dashboard.metricPages": "Pages",
   "dashboard.popularSeries": "Popular series",
   "dashboard.noSeries": "No series yet",
   "dashboard.unknown": "Unknown",