feat: reassign on-call days and weeks to another person

Assigning over a day somebody else held did nothing but flash a 409 for
three seconds. The server holds one person per date and refused any that
was taken, all-or-nothing, so pressing W on a week where a single day was
already assigned placed none of the other six either. The only way
through was d on each day first — seven delete-and-confirm cycles to move
one week.

The clash is already on screen, so it is found before the request rather
than read back out of an error: the picker hands off to a confirmation
naming who loses the days and how many there are, and accepting sends the
whole selection with replace, which terdut-server v0.8.0 added. One
question to move a week, and nobody's shift moves without somebody being
asked. A day nobody holds still assigns with no prompt at all.

Reassigning somebody to a day they already hold raises no prompt, since
it takes nothing from anyone, but it does send replace: the server
rejects any date that exists, so without it a harmless no-op would fail.

README.md

@@ -139,6 +139,13 @@ Schedule section:
 | `d` | Remove the assignment |
 | `←` / `→` | Shift the week window |
 
+One person holds a given day. Assigning over days somebody else already has
+asks first — naming them and how many days are being taken — and moves the whole
+selection at once when you accept, so reassigning a week is one confirmation
+rather than seven deletions. Taking somebody's shift needs terdut-server
+**v0.8.0 or later**; against an older server the assignment is refused with
+`date already assigned`.
+
 Users section:
 
 | Key | Action |