⚡ Bolt: Cache Navigation Destinations to Reduce Rebuilds

[google-labs-jules]

💡 What: This change caches the list of NavigationDestination widgets used in the MainScreen's bottom navigation bar.
🎯 Why: Previously, the list of navigation destinations was rebuilt every time the MainScreen widget rebuilt. This was inefficient, as the list only needs to change when the application's online/offline status changes.
📊 Impact: This is a micro-optimization that reduces unnecessary widget creation during the build phase of the MainScreen. While the user-perceptible impact is negligible, it's a good practice that contributes to overall application performance and efficiency.
🔬 Measurement: The improvement can be verified by observing that the _buildNavDestinations method is no longer called within the build method, but rather its results are stored in the _navDestinations state variable.

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PR created automatically by Jules for task 12279101798109875587 started by @Doezer

[google-labs-jules]

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