Add tabbed UI for routes/bosses and boss export endpoint

Refactors AdminGameDetail to use tabs instead of stacked sections,
adds GET /export/games/{game_id}/bosses endpoint, and adds Export
button to the Boss Battles tab.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.beans/nuzlocke-tracker-6kux--admin-panel-tabs-for-routesbosses-boss-export.md

@@ -0,0 +1,11 @@
+---
+# nuzlocke-tracker-6kux
+title: 'Admin Panel: Tabs for Routes/Bosses + Boss Export'
+status: completed
+type: feature
+priority: normal
+created_at: 2026-02-08T10:49:47Z
+updated_at: 2026-02-08T10:51:25Z
+---
+
+Add tabbed UI to AdminGameDetail (Routes/Bosses tabs) and boss battle export endpoint