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/components/Layout.tsx

@@ -28,6 +28,12 @@ export function Layout() {
               >
                 My Runs
               </Link>
+              <Link
+                to="/stats"
+                className="px-3 py-2 rounded-md text-sm font-medium hover:bg-gray-100 dark:hover:bg-gray-700"
+              >
+                Stats
+              </Link>
               <Link
                 to="/admin"
                 className="px-3 py-2 rounded-md text-sm font-medium hover:bg-gray-100 dark:hover:bg-gray-700"
@@ -87,6 +93,13 @@ export function Layout() {
               >
                 My Runs
               </Link>
+              <Link
+                to="/stats"
+                onClick={() => setMenuOpen(false)}
+                className="block px-3 py-2 rounded-md text-base font-medium hover:bg-gray-100 dark:hover:bg-gray-700"
+              >
+                Stats
+              </Link>
               <Link
                 to="/admin"
                 onClick={() => setMenuOpen(false)}