feat: add core watch interfaces and device stubs

[fabOnReact]

Summary

• add core TypeScript interfaces for connectors, data sync, and event handlers
• scaffold device directories for Garmin, Mi Band, Huawei, WearOS, and Apple Watch adapters
• update entrypoint to expose core interfaces and device adapters

Testing

• yarn lint (fails: ESLint couldn't find plugin "eslint-plugin-ft-flow")
• yarn add -D eslint-plugin-ft-flow (fails: Errors happened when preparing the environment)
• yarn test (fails: Cannot find module '@react-native/babel-preset')
• yarn add -D @react-native/babel-preset (fails: Errors happened when preparing the environment)
• yarn typecheck

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https://chatgpt.com/codex/tasks/task_e_68b7026ff0fc83209ec3f15673729d32