refactor(dialog): improve composition model and accessibility

Refactor Dialog 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**

- Simplify Dialog.Title / Dialog.Content / Dialog.Actions hierarchy
- Improve keyboard and accessibility handling
- Improve MD3 spacing and tonal elevation
- Reduce excessive wrappers

**Proposed API Direction**

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**Performance Opportunities**

- Reduce nested layout complexity
- Improve portal interaction handling
- Reduce unnecessary rerenders during visibility updates