[Feature]: Project PM assignment + deletion

[kiranfin]

User Story

As an Admin, I want to delete projects and assign/remove a project manager (PM) directly in the frontend, so that outdated projects stop using unnecessary space and every project has a clear, single owner.

Context & Motivation

• Frontend sub-task of the project-lifecycle story (parent).
• Covers the admin-facing UI for two of the parent's acceptance criteria: "Admin can delete a project" and "Admin can assign a PM to a project (and remove)" — plus the "exactly one PM per project" rule as enforced by the UI.
• Backend endpoints already exist and are wired in projectService (deleteProject, setProjectManager, clearProjectManager, getManagerCandidates).
• Branch: feature/254-projects-selection.

Acceptance Criteria

• Admin can delete a project from the frontend (project details drawer).
• Deletion is guarded by a confirmation dialog that names the project and its user-assignment count and states the action is irreversible.
• On successful deletion the project is removed from the list and the drawer closes; deletion errors are surfaced inline.
• Admin can assign a PM to a project via the frontend.
• Admin can remove/replace the assigned PM via the frontend.
• Only one PM can be assigned at a time; the UI overwrites rather than appends, and only PM/ADMIN-eligible users are selectable as manager.
• Delete and PM-assignment controls are only shown to admins (ADMIN permission group / canManageLifecycle); non-admins never see them.