Add system-wide IcoMoon to Font Awesome 7 compatibility layer

Replace the partial 10-icon mapping with a comprehensive layer
that covers 80+ Joomla IcoMoon icon-* classes. All icon-* prefixed
elements now render using Font Awesome 7 Free glyphs via ::before
pseudo-elements with the correct font-family, weight, and
smoothing. Covers navigation, actions, objects, status, media,
layout, and publishing icons.

Also fix back-to-top icon in error.php (icon-arrow-up → fa-solid
fa-arrow-up).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/error.php

@@ -418,7 +418,7 @@ $wa->useScript('user.js');         // js/user.js
 
 <?php if ($this->params->get('backTop') == 1) : ?>
 	<a href="#top" id="back-top" class="back-to-top-link" aria-label="<?php echo Text::_('TPL_MOKOCASSIOPEIA_BACKTOTOP'); ?>">
-		<span class="icon-arrow-up icon-fw" aria-hidden="true"></span>
+		<span class="fa-solid fa-arrow-up" aria-hidden="true"></span>
 	</a>
 <?php endif; ?>