[FE] Use feature branch to gate manager access toggle until backend is ready

[trillium]

Overview

No PostHog feature flag infrastructure is in place yet. Manager access toggle UI work (#2149) should land on a dedicated feature branch so it can be merged to development only once the backend data layer is finalized (#2155).

Action Items

• Create branch feature/manager-access-toggle off of development
• All UI work for the manager access toggle (EPIC: As a manager, I want a Toggle Feature so that I can Enable Manager Access #2149) targets this branch — not development directly
• Branch merges to development when the backend endpoint for querying project memberships is agreed upon and complete (ProjectTeamMember schema is the right join table but is bypassed by redundant fields on User and Project #2155)

Resources/Instructions

• Parent epic: EPIC: As a manager, I want a Toggle Feature so that I can Enable Manager Access #2149
• Backend data layer context: ProjectTeamMember schema is the right join table but is bypassed by redundant fields on User and Project #2155