refactor(appbar): modernize Appbar to latest MD3 spec

[matkoson]

Summary

Modernizes the Appbar components to align with the latest Material Design 3 (MD3) App bar specification.

Changes

• Appbar / AppbarHeader / AppbarContent / AppbarAction: MD3 token-driven layout, elevated/flat modes, and updated spacing/typography behavior
• New tokens.ts: shared MD3 App bar tokens for consistent theming
• Example: updated AppbarExample to demonstrate current MD3 patterns
• Docs (6.x): Appbar component docs + componentDocs6x / theme color data updates
• Tests: expanded unit coverage and snapshot updates for the new API/behavior

Out of scope

• No private/self-hosted CI workflows (kept on private fork only)

Test plan

• Appbar unit tests pass
• Example app Appbar screens look correct on iOS and Android
• Docs preview for 6.x Appbar pages
• Visual check against MD3 App bar spec

[CatLover01]

For the release of v6, I'm pretty sure we do not want to keep deprecated components / notices, remove them completly.

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[JKobrynski]

@matkoson @pawelkata you're both rewriting Appbar, and 11 files overlap, including every component and the snapshot. Could you agree who takes it? Worth noting that #5039 still edits the generated docs that #5094 deleted.

[matkoson]

Closed in favour of #5075, context: #5075 (comment)