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/tui/update.go

@@ -822,6 +822,7 @@ func (m Model) handleConfirmKey(msg tea.KeyMsg) (Model, tea.Cmd) {
 		}
 		m.pendingDeleteID = 0
 		m.pendingDeleteEntry = nil
+		m.pendingAssign = nil
 		return m, nil
 	}
 
@@ -851,6 +852,17 @@ func (m Model) handleConfirmKey(msg tea.KeyMsg) (Model, tea.Cmd) {
 		m.mode = modeDashboard
 		m.usersLoading = true
 		return m, deleteUserCmd(m.client, userID)
+
+	case confirmReassignSchedule:
+		p := m.pendingAssign
+		m.mode = modeDashboard
+		m.pendingAssign = nil
+		if p == nil {
+			return m, nil
+		}
+		m.scheduleLoading = true
+		return m, assignScheduleCmd(m.client, p.userID, p.dates, true,
+			m.scheduleWindow, m.scheduleWindow.AddDate(0, 0, 6))
 	}
 
 	return m, nil
@@ -899,15 +911,84 @@ func (m Model) handleUserPickerKey(msg tea.KeyMsg) (Model, tea.Cmd) {
 		} else {
 			dates = []string{d.Format("2006-01-02")}
 		}
+
+		// The server refuses a date somebody else holds, so ask before taking
+		// it rather than letting the request come back 409. The answer is
+		// already on screen — no round trip is needed to work out who loses
+		// their shift.
+		taken, holders := m.scheduleConflicts(dates, user.ID)
+		if len(taken) > 0 {
+			m.pendingAssign = &pendingAssign{
+				userID:   user.ID,
+				username: user.Username,
+				dates:    dates,
+				taken:    taken,
+				holders:  holders,
+			}
+			m.confirmTarget = confirmReassignSchedule
+			m.mode = modeConfirm
+			return m, nil
+		}
+
 		m.mode = modeDashboard
 		m.scheduleLoading = true
-		return m, assignScheduleCmd(m.client, user.ID, dates,
+		// Nobody else loses anything, but the server rejects any date that
+		// already exists — including days this same person already holds, which
+		// is a no-op worth letting through silently.
+		return m, assignScheduleCmd(m.client, user.ID, dates, m.scheduleOccupied(dates),
 			m.scheduleWindow, m.scheduleWindow.AddDate(0, 0, 6))
 	}
 
 	return m, nil
 }
 
+// scheduleConflicts reports which of dates are already held by somebody other
+// than newUserID, and the distinct names holding them.
+//
+// Days the target already owns are not conflicts — reassigning somebody to
+// their own shift takes nothing from anyone, and prompting for it would be
+// noise. The server still needs replace for those, since it rejects any date
+// that exists.
+func (m Model) scheduleConflicts(dates []string, newUserID int64) (taken, holders []string) {
+	held := make(map[string]api.ScheduleEntry, len(m.scheduleDays))
+	for _, d := range m.scheduleDays {
+		if d.entry != nil {
+			held[d.entry.Date] = *d.entry
+		}
+	}
+	seen := make(map[string]bool)
+	for _, date := range dates {
+		e, ok := held[date]
+		if !ok || e.UserID == newUserID {
+			continue
+		}
+		taken = append(taken, date)
+		if !seen[e.Username] {
+			seen[e.Username] = true
+			holders = append(holders, e.Username)
+		}
+	}
+	return taken, holders
+}
+
+// scheduleOccupied reports whether any of dates already has an entry at all,
+// including one belonging to the incoming user. That is what decides whether
+// the request needs replace, as opposed to whether it needs confirming.
+func (m Model) scheduleOccupied(dates []string) bool {
+	held := make(map[string]bool, len(m.scheduleDays))
+	for _, d := range m.scheduleDays {
+		if d.entry != nil {
+			held[d.entry.Date] = true
+		}
+	}
+	for _, date := range dates {
+		if held[date] {
+			return true
+		}
+	}
+	return false
+}
+
 // ── User management ───────────────────────────────────────────────────────────
 
 func (m Model) handleUserCreateKey(msg tea.KeyMsg) (Model, tea.Cmd) {