feat: add overlayParentActions() for stacked action modals

[leek]

Summary

• Adds an opt-in overlayParentActions() method on child actions that keeps the parent modal visible when the child opens on top
• Without the flag, the default close/reopen behavior is unchanged
• CSS raises stacked child modals to z-50 (above parent's z-40) and lightens the child backdrop to avoid double-darkness

Supersedes #19163. Resolves #19166.

Usage

Action::make('editItems')
    ->slideOver()
    ->schema([
        Repeater::make('items')
            ->deleteAction(
                static fn (Action $action) => $action->overlayParentActions(),
            ),
    ])
    ->action(function () {
        // ...
    })

Changes

File	What
HasParentActions.php	New $shouldOverlayParentActions property, overlayParentActions() setter, shouldOverlayParentActions() getter
InteractsWithActions.php	syncActionModals() dispatches overlayParentActions flag from mounted action
modals.js	Skip closeModal() only when flag is true AND nesting depth is increasing
modal.css	z-50 stacking + lighter child backdrop for sibling .fi-modal-open elements
02-modals.md	New "Overlaying child actions on top of parent actions" docs section

Test plan

• Open a slide-over with a repeater, click delete on an item with ->overlayParentActions() → confirmation dialog stacks on top, slide-over stays visible
• Same scenario without ->overlayParentActions() → default close/reopen behavior unchanged
• Dismissing the child action returns focus to the still-open parent
• composer test:phpstan passes (pre-existing env issue on local, CI should be clean)