Frontend: Mount face-marker overlay in read-only display mode for viewers

Drops shouldShowEditButton() from the <p-face-marker-overlay> v-if so
non-editable users (Plus / Pro viewer role, anyone reaching the eye
toggle in the sidebar) actually see marker rectangles AND the
in-overlay Back button when they activate display mode. The previous
gate required edit permission, so viewers entered face-marker mode
(chrome hidden), saw nothing, and had no way to exit. Edit gestures
stay gated separately by the overlay's isEditMode flag and the
lightbox-side onCreateFaceMarker / onRemoveFaceMarker /
toggleFaceMarkerEdit handlers that re-check shouldShowEditButton().
Pre-existing bug; surfaced while testing the recent ACL-driven
sidebar refactor (#4966).

frontend/src/component/lightbox.vue

@@ -44,7 +44,7 @@
       >
         <div ref="lightbox" tabindex="-1" class="p-lightbox__pswp no-transition"></div>
         <p-face-marker-overlay
-          v-if="shouldShowEditButton() && featPeople && faceMarkers.active && pswp()"
+          v-if="featPeople && faceMarkers.active && pswp()"
           ref="faceMarkerOverlay"
           :mode="faceMarkers.mode"
           :markers="markers"