Labor rate edits that rename, and deletes that talk back

Two fixes in Settings → Labor Rates:

• Editing a rate now edits the rate. Changing a row's classification or effective date used to save your change as a new row and leave the old one sitting there — which is how you'd end up with two of the same classification, one of them still marked "in use". Edits now update the row you're on. If your edit would collide with a rate that already exists for that classification and date, it says so instead of guessing.
• A delete that can't happen now says why. Deleting a rate (or a burden config) that the database refuses — for example, a rate an NTP-locked budget baseline still references — used to fail silently: click, nothing. You now get the reason on screen, including what to unlock first.