Code Quality: Prevent exception when AppModel.LastSelectedTabIndex is set before MainPage is loaded

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Resolved / Related Issues

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• Closes FILES-APP-3NE

Steps used to test these changes

Stability is a top priority for Files and all changes are required to go through testing before being merged into the repo. Please include a list of steps that you used to test this PR.

1. Opened Files after applying the changes.
2. Ensured that the app launches without crashing when the AppModel.LastSelectedTabIndex is set before the MainPage is fully loaded.
3. Verified that tab selection works correctly after the MainPage is fully loaded.
4. Triggered the scenario that caused the InvalidCastException before the fix and confirmed that the exception is no longer thrown.

Fixes FILES-APP-3NE. The issue was that: AppModel's tab selection logic attempts to cast Frame.Content to MainPage while SplashScreenPage's Grid is still active, causing InvalidCastException.

• Added a check to ensure that rootFrame.Content is a MainPage instance before accessing its ViewModel.
• This prevents a NullReferenceException that could occur if AppModel.LastSelectedTabIndex is set before the MainPage is fully loaded.

This fix was generated by Seer in Sentry, triggered by Yair. 👁️ Run ID: 502085

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