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/pages/index.ts

@@ -2,3 +2,4 @@ export { Home } from './Home'
 export { NewRun } from './NewRun'
 export { RunList } from './RunList'
 export { RunEncounters } from './RunEncounters'
+export { Stats } from './Stats'