refactor(invite): decouple global session state management side-effects from useValidateInviteQuery hook

Current Behavior

In apps/meteor/client/views/invite/hooks/useValidateInviteQuery.ts, there is an outstanding architectural technical debt marker:
// FIXME: decouple this state management from the query

Currently, useValidateInviteQuery dispatches global session state side-effects (setLoginDefaultState) directly inside its asynchronous queryFn. This violates React Query anti-patterns, blurring the lines between pure data-fetching and UI state mutation, making the hook harder to test and reuse.

Expected Behavior

The setLoginDefaultState side-effects should be extracted completely from the useValidateInviteQuery fetcher logic. Instead, the UI component consuming the data (InvitePage.tsx) should track the query's isSuccess/data state and dispatch the session updates deterministically via a useEffect.

Tasks

• Extract setLoginDefaultState and Accounts_RegistrationForm checks out of useValidateInviteQuery
• Migrate the route logic strictly into InvitePage.tsx bound to the query lifecycle
• Remove // FIXME: decouple this state management from the query

Start with apps/meteor/client/views/invite/hooks/useValidateInviteQuery.ts and apps/meteor/client/views/invite/InvitePage.tsx. Trace the query lifecycle and the existing setLoginDefaultState and Accounts_RegistrationForm checks, then verify that fetching is separated from session updates and the route handles them on successful query data.