Mark-as-Completed: agency approval gate before completion

Improve the request-completion flow so the agency keeps control of when a request is closed.

Asks consolidated here:

• Agency approval gate — when a client clicks 'Mark as Completed', prompt the agency for approval (and optionally final time-tracking entries) before the request actually closes. Options to remove the button entirely or rename it.
• Lock comments on closed requests — already shipped (see comment below).