fix(angular): stop a selected field-picker option rendering white-on-panel

[duguankui]

Description

A selected option in the field picker's dropdown rendered white text on the panel background once
the mouse left it. Same defect, same shape, as dignite-projects/abp-modules#222 — this picker carried
an identical copy of the rule.

The rule meant to give a selected option a --lpx-brand fill carried !important on its colour but
not on its background:

::ng-deep .field-picker-dropdown .ant-select-item-option-selected:not(.ant-select-item-option-disabled) {
  background-color: var(--lpx-brand);        // ← no !important
  color: var(--bs-white) !important;
}

while the .ant-select-item rule below it zeroes every option background with !important, to stop
ant-design's own idle-state default painting through. !important beats specificity, so the
background never applied and the white text always did — invisible while the panel was still #fff,
merely illegible once it started following --bs-secondary-bg. The hover rule directly above has
!important on both halves, which is why hovering a selected option looked right and moving off it
did not.

Rather than adding the missing !important, the rule is removed. A selected option now reads
exactly like an unselected one, with ant-design's own checkmark (.ant-select-item-option-state) as
the indicator. The picker is nzMode="multiple" — several options are selected at once, so filling
each of their rows competes with the hover state for attention rather than adding information.

The page-parent picker is untouched: it is single-select, and its own selected-node rule already
carries !important on both halves, so it never had this problem.

How to test it?

Open the content-type editor's field picker, select two or three fields, and move the mouse off the
dropdown. Every option should read the same, with checkmarks marking the selected ones; hovering
should still give the blue --bs-primary row with white text.

ng build site passes. The compiled output no longer carries the selected-option rule and keeps the
panel, hover and idle-option rules unchanged. No tests added — this is emitted CSS, and the repo has
no stylesheet coverage to extend.