Default copy and confirmation text for lower-priority mode

The lower-priority usage-limit mode carries its own built-in wording: label "Continue now at lower priority", status "Working at lower priority until {reset}", wait banner "Working at lower priority · waiting for capacity", and "You've used this week's lower-priority allowance". A confirmation is assembled at runtime around "Continuing now at lower priority" and ". Your weekly limit still applies, and responses may pause while waiting for spare capacity. Run /low-priority to stop." Every string can be replaced by the server config, and the whole thing stays off unless that config sets enabled to true.