feat(backoffice): add dashboard statistics with charts

Add GET /stats API endpoint with collection overview, reading status,
format/library breakdowns, top series, and monthly additions.
Replace static home page with interactive dashboard featuring donut
charts, bar charts, and progress bars. Use distinct colors for series
(warning/yellow) across nav, page titles, and quick links.

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

apps/backoffice/app/components/ui/Icon.tsx

@@ -90,7 +90,7 @@ const colorClasses: Partial<Record<IconName, string>> = {
   libraries: "text-primary",
   jobs: "text-warning",
   tokens: "text-error",
-  series: "text-primary",
+  series: "text-warning",
   settings: "text-muted-foreground",
   image: "text-primary",
   cache: "text-warning",