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.

internal/api/client.go

@@ -358,11 +358,17 @@ func (c *Client) GetCurrentOnCall() (*ScheduleEntry, error) {
 	return &entry, nil
 }
 
-func (c *Client) AssignSchedule(userID int64, dates []string) ([]ScheduleEntry, error) {
+// AssignSchedule puts one user on call for the given dates.
+//
+// The server holds one person per day and refuses a date somebody already has,
+// so replace is what takes a shift off its current holder. It is all-or-nothing
+// either way: a week of free and taken days moves as a unit, or not at all.
+func (c *Client) AssignSchedule(userID int64, dates []string, replace bool) ([]ScheduleEntry, error) {
 	body := struct {
-		UserID int64    `json:"user_id"`
-		Dates  []string `json:"dates"`
-	}{UserID: userID, Dates: dates}
+		UserID  int64    `json:"user_id"`
+		Dates   []string `json:"dates"`
+		Replace bool     `json:"replace,omitempty"`
+	}{UserID: userID, Dates: dates, Replace: replace}
 	req, err := c.newRequestWithBody(http.MethodPost, "/api/schedule", body)
 	if err != nil {
 		return nil, err