Add stats screen with backend endpoint and frontend page

Implements a dedicated /stats page showing cross-run aggregate statistics:
run overview with win rate, runs by game bar chart, encounter breakdowns,
top caught/encountered pokemon rankings, mortality analysis with death
causes, and type distribution. Backend endpoint uses aggregate SQL queries
to avoid N+1 fetching.

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

frontend/src/hooks/useStats.ts

@@ -0,0 +1,9 @@
+import { useQuery } from '@tanstack/react-query'
+import { getStats } from '../api/stats'
+
+export function useStats() {
+  return useQuery({
+    queryKey: ['stats'],
+    queryFn: getStats,
+  })
+}