🧪 Prevent data race with UI tests

The change to using a global variable containing grapheme cluster width
overrides introduced a data race.

Disabling parallel tests in the UI package will prevent this from being
detected. While this does not solve the issue, the global variable is
only set once before any rendering and all reads will be safe.

internal/ui/ui_test.go

@@ -772,8 +772,6 @@ func TestModel(t *testing.T) {
 		tt := tt
 
 		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
 			c := testState()
 
 			if tt.args.state != nil {