feat: accept-invite request/response contract for volunteer account linking

[arturasmckwcz]

Why

Part of the epic: need4deed-org/be#922 — inviting existing Volunteer rows (people who
submitted the public volunteer form but never created a login) to register as
a User with role: VOLUNTEER.

Backend will mint an invite JWT (type: "invite") tied to a pre-created,
inactive User row, and email a link. The frontend needs a page where the
invited person sets a password; that page calls a new backend endpoint. This
issue defines that endpoint's request/response contract in the SDK, since per
org convention (shared-rules.md) the SDK is the source of truth for the API
surface and must be published before be/fe consume it.

Scope

Add to src/types/api/ (follow existing file/naming conventions — check
whether an auth.ts file already exists for auth-related bodies, e.g.
password-reset, before deciding where this lives):

• An accept-invite request body type, e.g. ApiAcceptInviteBody —
  { token: string; password: string; confirmPassword: string }. Match the
  field naming/casing of whatever existing password-reset body type already
  exists in the SDK exactly (don't invent new naming for the same concept).
• A response type — likely reuse ApiUserGet if the endpoint logs the person
  in / returns the activated account. Confirm against whatever be actually
  returns once its issue below is scoped.
• Export the new type(s) from src/types/api/index.ts.

Open questions to resolve with the be dev during TODO review

• Final endpoint path/method chosen in <BE_INVITE_ISSUE_URL> (extending
  POST /auth/password-reset vs a new POST /auth/accept-invite) — confirm
  before finalizing type names.
• Whether a new Endpoint constant should be added to
  src/constants/endpoints.ts for consistency with the existing
  Endpoint.REGISTER.

Acceptance criteria

• New type(s) added and exported from src/types/api/index.ts
• Version published to npm (automatic on merge to main via
  .github/workflows/publish.yml)
• be/fe issues below can pin the new published version