feat: add CssScrollPadding, CssOverscrollBehavior, CssAppearance

[kevin-sakemaer]

Summary

• Add CssScrollPadding sealed class for scroll-padding with .all(), .symmetric(), .only() factories (closes Add scroll-padding CSS property support to Style #96)
• Add CssOverscrollBehavior sealed class for overscroll-behavior with .auto, .contain, .none keywords and .xy() two-value syntax (closes Add overscroll-behavior CSS property support to Style #97)
• Add CssAppearance sealed class for appearance with .none, .auto, .menulistButton, .textfield keywords (closes Add appearance CSS property support to Style #98)

All types include .variable(), .raw(), and .global() escape hatches and are wired into Style.typed().

Test plan

• All 3 new type classes have dedicated test files with 100% coverage
• dart analyze passes with no issues
• All 633 tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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