PLIA-3972: Load front-end overlay for any user who can edit content

[rdom-si]

Problem

A customer reported that on preview pages the floating Siteimprove overlay never loads, the Prepublish toolbar button does nothing (just appends # to the URL), and no siteimprove.js is present in the DOM — while everything works fine in wp-admin.

Root cause

siteimprove_wp_head() was the only permission check in the plugin gated on hardcoded role names:

$allowed_roles = array( 'shop_manager', 'contributor', 'author', 'editor', 'administrator' );
if ( array_intersect( $allowed_roles, $user->roles ) ) { ... }

Every other path (siteimprove_init(), the settings screens) uses current_user_can( ... ). Users whose role name isn't in that list — custom/renamed roles, or multisite super admins not explicitly assigned a role on a subsite — have editing capabilities but no matching role name, so the overlay JS was never enqueued on the front end. The admin-side paths are capability-gated, which is why they kept working.

This also explains the inert Prepublish button: add_prepublish_toolbar_item() renders it without a role check, but its click handler lives in the un-loaded siteimprove.js, so clicking just follows href="#".

Fix

Gate the front-end injection on current_user_can( 'edit_posts' ), matching the capability check already used for the identical 'siteimprove_input' enqueue in siteimprove_init(). This covers custom roles, renamed roles, and super admins while still excluding subscribers/visitors.

Version bumped 2.1.2 → 2.1.3 (patch) with changelog entries in readme.txt and README.md.

Testing notes

Reproduce with a user who has edit_posts but a non-standard role (e.g. a custom role, or a multisite super admin not added to the subsite): before the fix the overlay/Prepublish button is missing on previews; after, both load. Standard roles (administrator/editor/author/contributor/shop_manager) and subscribers are unaffected.