fix(app): clear viewed session notifications

[Brendonovich]

Summary

• mark existing session notifications viewed when the new layout opens a session route
• wait for persisted notification state before clearing unread indexes
• restore the route-entry behavior provided by the legacy layout

Testing

• bun typecheck from packages/app
• bun test --preload ./happydom.ts ./src/utils/session-route.test.ts ./src/utils/notification-click.test.ts
• pre-push repository typecheck (23 packages)