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/api/stats.ts

@@ -0,0 +1,6 @@
+import { api } from './client'
+import type { StatsResponse } from '../types/stats'
+
+export function getStats(): Promise<StatsResponse> {
+  return api.get('/stats')
+}