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/series/page.tsx

@@ -46,7 +46,7 @@ export default async function SeriesPage({
     <>
       <div className="mb-6">
         <h1 className="text-3xl font-bold text-foreground flex items-center gap-3">
-          <svg className="w-8 h-8 text-primary" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+          <svg className="w-8 h-8 text-warning" fill="none" stroke="currentColor" viewBox="0 0 24 24">
             <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M19 11H5m14 0a2 2 0 012 2v6a2 2 0 01-2 2H5a2 2 0 01-2-2v-6a2 2 0 012-2m14 0V9a2 2 0 00-2-2M5 11V9a2 2 0 012-2m0 0V5a2 2 0 012-2h6a2 2 0 012 2v2M7 7h10" />
           </svg>
           Series