Feature Request: Provide built-in notifications for account state changes

### Is your feature request related to a problem? Please describe.

Apps built with the Base Account SDK often need to know when account data changes, such as when balances update or an account finishes initializing. Right now, developers usually have to re-fetch data or add their own polling to detect these changes.

## Problem Description
Because there are no clear signals for account updates, applications may end up making extra requests or maintaining custom logic just to stay in sync. This adds complexity and makes it easier for account state to become stale.

### Describe the solution you'd like
Add optional hooks or event-style APIs that notify consumers when account state changes, such as balance updates or initialization completion. The feature should be lightweight, additive, and consistent with existing SDK behavior.

No files, tests, or entry points are identified. Start by reviewing the existing account state APIs and SDK behavior, then define the notification hooks or event-style API, its account-change semantics, and tests covering balance updates and initialization completion.

Written by the indexing model from the issue text.