refactor(platform-navigation): isolate native navigation chrome

[eeee0717]

Summary

• move root stack, tab bar, message header, global search, zoom transition, and header inset differences into platform modules
• keep route files and shared screen state platform-neutral
• preserve Android image tabs and iOS native navigation/search behavior

Validation

• targeted oxlint and Expo lint
• pnpm typecheck:app
• navigation, search, chat workspace, painting composer, and effort layout suites passed

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