refactor(snackbar): modernize layout and action API

[ruben-rebelo]

Refactor SnackBar to improve Material Design 3 compliance, remove problematic children patterns, improve performance characteristics, and modernize the public API.

Problems

• Current API relies heavily on children composition in ways that are difficult to optimize
• Internal layout logic is tightly coupled to arbitrary child structures
• MD3 behaviors and visuals are not fully aligned with current Material guidelines
• Rendering performance can degrade in large lists or frequently updated screens
• Type safety and discoverability of the API can be improved
• The component has inconsistent behavior between platforms and edge cases

Focus Areas

• Improve action rendering model
• Improve responsiveness and multiline behavior
• Improve MD3 tonal surface handling
• Improve animation consistency

Proposed API Direction

<Snackbar visible message="Changes saved" action={{ label: 'Undo', onPress }} />

Performance Opportunities

• Reduce animated subtree complexity
• Improve portal rendering behavior
• Reduce unnecessary visibility transitions