fix: useTracker should only render once on mount (closes #439)

[nicolv]

Summary

Closes #439 - useTracker renders two times at mount

Problem

The previous implementation of useTrackerNoDeps would:

1. Stop and recreate the Tracker computation on every render
2. Combined with React 18 StrictMode double-rendering, this caused the component to render more times than expected

Solution

1. Added useSyncEffect hook - A custom hook that runs effects synchronously during render but defers cleanup by 1 second to avoid StrictMode double-rendering issues

2. Simplified useTrackerNoDeps - Now delegates to useTrackerWithDeps instead of having its own complex render-phase logic

3. Updated useTrackerWithDeps - Uses useSyncEffect to manage the Tracker computation lifecycle, avoiding the stop/recreate on every render

Changes

• packages/react-meteor-data/useSyncEffect.ts - New file: the useSyncEffect hook
• packages/react-meteor-data/useTracker.ts - Simplified to use useSyncEffect

Key Technical Points

• The previous code used Meteor.defer() and render-phase computation creation, which caused issues with StrictMode
• useSyncEffect runs the effect in useMemo (during render) but delays cleanup by 1 second, avoiding StrictMode double-render issues
• This is the same approach as PR [WIP] simpler useTracker #436 but refined

Testing

The existing noDepsTester test in useTracker.tests.js already covers StrictMode behavior.

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Related to #439