[iOS] Fix Dynamic Type font auto-scaling not updating at runtime

[Copilot]

FontManager caches UIFont instances keyed on the Font struct. When the user changes the system Dynamic Type size at runtime, the cache key is unchanged, so stale pre-scaled fonts are returned until the app restarts.

Changes

• FontManager.iOS.cs — Register a ContentSizeCategoryChanged observer that clears _fonts, ensuring the next GetFont call re-invokes UIFontMetrics.DefaultMetrics.GetScaledFont with the current category.

• PageViewController.cs — In TraitCollectionDidChange, detect PreferredContentSizeCategory changes and drive a recursive font invalidation pass:

if (previousTraitCollection is not null &&
    previousTraitCollection.PreferredContentSizeCategory != TraitCollection.PreferredContentSizeCategory)
{
    InvalidateFontsOnContentSizeChanged(CurrentView as IView);
}

• InvalidateFontsOnContentSizeChanged — Walks the visual tree via IVisualTreeElement.GetVisualChildren(). For any ITextStyle view with Font.AutoScalingEnabled: true and a live handler, calls UpdateValue(nameof(ITextStyle.Font)) (re-fetches from the now-cleared cache) and InvalidateMeasure().

Known limitation: Virtualized items in CollectionView/ListView that are off-screen are not updated in-flight; they pick up the correct font on next cell reuse. Shell-based navigation pages are not covered by this path and may require separate handling.

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