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_test.go

@@ -166,6 +166,28 @@ func TestClient_RequestBodies(t *testing.T) {
 		}
 	})
 
+	// replace is what takes a day off its current holder, so it has to reach the
+	// wire when asked for — and stay off it when not.
+	t.Run("assign schedule", func(t *testing.T) {
+		c, got := stub(t, http.StatusCreated, `[]`)
+		if _, err := c.AssignSchedule(3, []string{"2026-07-27"}, false); err != nil {
+			t.Fatalf("assign: %v", err)
+		}
+		if got.body != `{"user_id":3,"dates":["2026-07-27"]}` {
+			t.Errorf("unexpected body %q", got.body)
+		}
+	})
+
+	t.Run("assign schedule with replace", func(t *testing.T) {
+		c, got := stub(t, http.StatusCreated, `[]`)
+		if _, err := c.AssignSchedule(3, []string{"2026-07-27"}, true); err != nil {
+			t.Fatalf("assign: %v", err)
+		}
+		if got.body != `{"user_id":3,"dates":["2026-07-27"],"replace":true}` {
+			t.Errorf("unexpected body %q", got.body)
+		}
+	})
+
 	t.Run("set notify target", func(t *testing.T) {
 		c, got := stub(t, http.StatusOK, `{}`)
 		if _, err := c.SetUserNotifyTarget(3, "terdut-niklas"); err != nil {