refactor(platform-ui): isolate native presentation primitives

[eeee0717]

Summary

• move app icon glyphs, built-in tool visuals, message follow behavior, and effort backdrops behind platform files
• keep shared state and animation ownership in platform-neutral modules
• replace runtime Platform.OS mutation tests with direct platform-boundary coverage

Validation

• targeted oxlint and Expo lint
• pnpm typecheck:app
• app icon, message list, tool presentation, and tool part suites: 38 passing tests

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