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/api/src/openapi.rs

@@ -36,6 +36,7 @@ use utoipa::OpenApi;
         crate::tokens::create_token,
         crate::tokens::revoke_token,
         crate::tokens::delete_token,
+        crate::stats::get_stats,
         crate::settings::get_settings,
         crate::settings::get_setting,
         crate::settings::update_setting,
@@ -78,6 +79,14 @@ use utoipa::OpenApi;
             crate::settings::ClearCacheResponse,
             crate::settings::CacheStats,
             crate::settings::ThumbnailStats,
+            crate::stats::StatsResponse,
+            crate::stats::StatsOverview,
+            crate::stats::ReadingStatusStats,
+            crate::stats::FormatCount,
+            crate::stats::LanguageCount,
+            crate::stats::LibraryStats,
+            crate::stats::TopSeries,
+            crate::stats::MonthlyAdditions,
             ErrorResponse,
         )
     ),