refactor(platform-settings): isolate native settings chrome

[eeee0717]

Summary

• move assistant, data, permission, provider search, and model search chrome into paired iOS and Android modules
• retain shared filtering, selection, routing, and mutation state in the owning screens
• use children-based composition for reusable model search layouts

Validation

• targeted oxlint and Expo lint
• pnpm typecheck:app
• provider model, permission, and related focused suites passed

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