feat(salary): BK-29 merge the team rate modal into a single team rates modal

Both modals wrote hourly rates for the same team from the same effective date, so splitting them across two buttons made the user pick a mechanism before knowing which one applied. The flat rate is now a section of the grade rates modal, with one shared "effective from" date on top and the per-grade rollout below it.

Blocks inside the modal are separated by a hairline rule instead of cards: boxes inside the modal box would read as separate windows.