Fix: Defer Omnibar padding calculation until UI elements are measured

[seer-by-sentry]

Resolved / Related Issues

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• Closes FILES-APP-4D1

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. Observed the issue reported in FILES-APP-4D1 where window resize triggers Omnibar mode change, using unmeasured UI element dimensions (ActualWidth=0) in padding calculations, causing WinUI's layout system to divide by zero.
2. Verified that the fix prevents division by zero errors in WinUI's layout system when calculating AutoSuggestBox padding.
3. Confirmed that padding calculation is deferred to the dispatcher queue with low priority.
4. Ensured that a layout update is forced to make ActualWidth values available before calculation.
5. Validated that padding is only updated if valid measurements are obtained.
6. Resized the application window multiple times to ensure the issue is resolved and no crashes occur.

This fix was generated by Seer in Sentry, triggered by Yair. 👁️ Run ID: 1202654

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