feat(web,a11y): standardize the FullScreenModal UI (#8566)

* feat(web,a11y): standardize the FullScreenModal look

* consistent header, padding, close button, and radius as BaseModal
* vertically stacking ConfirmDialogue CTA buttons in narrow screens
* adding aria-modal tags for screen reader
* add viewport-specific height limits on modals, to enable scrolling
* prevent focus from being hidden under sticky content in modals
* standardize FullScreenModal widths using a Prop

* wip: consistent padding with header

* fix: alignment on "create user" and "edit user" modals

* fix: horizontal modal content alignment

* fix: create user CTA buttons

* chore: remove unnecessary warning

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Co-authored-by: Jason Rasmussen <jrasm91@gmail.com>

web/src/lib/components/faces-page/merge-face-selector.svelte

@@ -161,6 +161,7 @@
 
     {#if isShowConfirmation}
       <ConfirmDialogue
+        id="merge-people-modal"
         title="Merge people"
         confirmText="Merge"
         onConfirm={handleMerge}