fix: add subscription manager to prevent GraphQL subscription leaks

[Flegma]

Summary

• Create useSubscriptionManager composable for centralized subscription lifecycle management
• Refactor NotificationStore (3 subscriptions), ApplicationSettings (3 subscriptions + setInterval guard), MatchmakingStore (3 subscriptions), MatchLobbyStore (8 subscriptions) to track and auto-cleanup subscriptions
• Add cleanup on auth state change (when me becomes falsy)
• Fix setInterval leak in ApplicationSettings where a new interval was created on every subscription emit

Closes 5stackgg/5stack-panel#385

Test plan

• Verify subscriptions are created on login and cleaned up on logout
• Verify re-authentication doesn't accumulate duplicate subscriptions
• Verify latency check interval is created only once
• Verify all store data updates correctly via subscriptions