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/App.tsx

@@ -1,7 +1,7 @@
 import { Routes, Route, Navigate } from 'react-router-dom'
 import { Layout } from './components'
 import { AdminLayout } from './components/admin'
-import { Home, NewRun, RunList, RunEncounters } from './pages'
+import { Home, NewRun, RunList, RunEncounters, Stats } from './pages'
 import {
   AdminGames,
   AdminGameDetail,
@@ -18,6 +18,7 @@ function App() {
         <Route path="runs" element={<RunList />} />
         <Route path="runs/new" element={<NewRun />} />
         <Route path="runs/:runId" element={<RunEncounters />} />
+        <Route path="stats" element={<Stats />} />
         <Route path="runs/:runId/encounters" element={<Navigate to=".." relative="path" replace />} />
         <Route path="admin" element={<AdminLayout />}>
           <Route index element={<Navigate to="/admin/games" replace />} />