[WC-3502]: fix gallery design mode preview blocking widget selection

[yordan-st]

Pull request type

Bug fix (non-breaking change which fixes an issue)

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Description

In Studio Pro design mode, widgets placed inside a gallery item could not be
hovered or selected — double-clicking an image or text opened the properties of
the surrounding container instead. The decorative gradient fade drawn by
.widget-gallery-content::before spans the full gallery content at z-index: 1
and, lacking pointer-events: none, captured all hit-testing, so Studio Pro
resolved clicks to the nearest ancestor.

Marked the pseudo-element as pointer-events: none. It is purely decorative, so
the fade still renders identically while clicks pass through to the real widgets
underneath. The stylesheet is imported only by Gallery.editorPreview.tsx, so
there is no runtime impact.

Ticket: https://mendix.atlassian.net/browse/WC-3502

No docs PR needed — no XML changes, no runtime behavior change. Changelog entry
added under [Unreleased].

What should be covered while testing?

Studio Pro 11.12.1 (the version the issue was reported against):

1. Create a new page from the List page template (it contains a Gallery).
2. Stay in Design mode — do not switch to Structure mode.
3. Double-click the image inside a gallery item → the Image properties dialog
   should open (before the fix: Container properties opened).
4. Repeat for the text widget inside the item → Text properties should open.
5. Hover over the nested widgets → the highlight should land on the widget itself,
   not on the container.
6. Confirm the gradient fade over the lower gallery rows still renders as before.
7. Confirm Structure mode and the Page Explorer still select widgets correctly.
8. Run the app and confirm the gallery renders and behaves unchanged at runtime.

Note: design-mode preview bundles are cached — press F4 (Synchronize project
directory) after deploying the widget, and reopen the app if the old behaviour
persists.