refactor(FR-3877): read myUserV2 at login through Relay so the response type is generated

The login read was a raw `client.query` string with a hand-written response
type. `RelayEnvironment`'s fetch function resolves `globalThis.backendaiclient`
at request time — the same signed `/admin/gql` request `client.query` makes —
and `connectViaGQL` installs the client before its first read, so the query
can be a `graphql` tag inside the function: Relay compiles it and generates
`loginSessionAuthMyUserQuery`, and the hand-written type goes away.

The test mocks the `RelayEnvironment` module with `createMockEnvironment()`
and queues one payload per page.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BX2RuEAvsXWdxgi2MdaMsz

Author: ironAiken2