users: replace js client with oats generated client.

Replace references to `import {client} from 'src/utils/api'` in the following files with `import {FUNCTION_NAME} from 'src/client'`

- `src/Signin.tsx`
- `src/shared/actions/me.ts`

Ensure sufficient tests are available before making the switch and that tests pass after replacement.

Start with src/Signin.tsx and src/shared/actions/me.ts, locating the imports from src/utils/api and the corresponding generated exports in src/client. Check that sufficient tests cover these paths before replacing the references, then run the relevant test suite; done means both files use the generated client and all tests pass.

Written by the indexing model from the issue text.