Add global events, map, cache and leaderboard screens

Introduce four new screens for global events: GlobalEventsScreen (lists public events and handles joining), GlobalMapScreen (map/list view of event caches with user location via expo-location and markers using react-native-maps), GlobalCacheViewScreen (cache detail view with ability to log discoveries), and GlobalLeaderboardScreen (rankings and recent finds). Screens use existing global hooks and SessionStore for fetching/joining events, caches, finds and for storing current global player context; they include loading/error states and navigation between screens. Also add simple tab components, styling, and refresh actions.

.gitignore

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 # Learn more https://docs.github.com/en/get-started/getting-started-with-git/ignoring-files
 
+# Environment variables (contains secrets)
+.env
+.env.local
+
 # dependencies
 node_modules/