[Feature] Implement web sync locks

[stevensJourney]

Overview

This uses the Navigator locks in the Mutex class from here powersync-ja/sqlite_async.dart#54 to limit the number of active sync implementations when multiple tabs are used.

This is only a stop-gap to prevent multiple tabs with multiple StreamingSyncImplementations from applying sync bucket changes at the same time (this can leave the local SQLite DB in an inconsistent or broken state).

This method enhances the safety of using multiple tabs, but is far from complete multiple tab support.

This should address the issue of "Flutter web + PowerSync still has the issue where multiple open tabs will cause sqlite to get messed up" from #11

Testing

Note that the app works when switching between tabs. The sync status is isolated per tab - only 1 tab is ever connected or uploading at a time.

locked_tabs.mp4

Todos:

• Update sqlite_async package

[rkistner]

The approach here looks good to me