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>

.beans/nuzlocke-tracker-9ngw--stats-screen.md

@@ -1,11 +1,11 @@
 ---
 # nuzlocke-tracker-9ngw
 title: Stats Screen
-status: todo
+status: completed
 type: feature
 priority: normal
 created_at: 2026-02-07T19:19:40Z
-updated_at: 2026-02-07T19:28:49Z
+updated_at: 2026-02-07T19:45:55Z
 ---
 
 A dedicated stats page aggregating data across all runs. Accessible from the main navigation.
@@ -56,12 +56,12 @@ Add `GET /api/stats` that runs aggregate queries server-side and returns pre-com
 - Responsive grid layout matching the existing design system (dark mode support)
 
 ## Checklist
-- [ ] Add `/stats` route and page component
-- [ ] Add "Stats" navigation link
-- [ ] Fetch all runs with encounters (or add backend stats endpoint)
-- [ ] Run Overview section (counts, win rate, duration)
-- [ ] Encounter Stats section (caught/fainted/missed breakdown)
-- [ ] Pokemon Rankings section (top caught, top encountered, expandable)
-- [ ] Team & Deaths section (mortality, death causes, type distribution)
-- [ ] Charts for region/generation/type breakdowns
-- [ ] Responsive layout + dark mode styling
+- [x] Add `/stats` route and page component
+- [x] Add "Stats" navigation link
+- [x] Fetch all runs with encounters (or add backend stats endpoint)
+- [x] Run Overview section (counts, win rate, duration)
+- [x] Encounter Stats section (caught/fainted/missed breakdown)
+- [x] Pokemon Rankings section (top caught, top encountered, expandable)
+- [x] Team & Deaths section (mortality, death causes, type distribution)
+- [x] Charts for region/generation/type breakdowns
+- [x] Responsive layout + dark mode styling