feat: hero variants, block colors, theme preview tab, CSS var sync on upgrade

- Add .hero#primary / .hero#secondary CSS variant system to template.css
- Add :nth-child() block color palette for top-a/b, bottom-a/b positions
- Add named per-module overrides (#block-highlight, #block-cta, #block-alert)
- Add all hero + block CSS variables to light/dark custom palette templates
- Add theme-test.html with full visual CSS variable reference + color test
- Add "Theme Preview" tab to admin config with embedded iframe of test sheet
- Add script.php install/update script (Joomla 5/6 compatible)
- Add sync_custom_vars.php — detects missing vars in user palettes on upgrade
- Add en-GB and en-US language strings for new admin config fields
- Update CSS_VARIABLES.md and CHANGELOG.md
- Bump version to 03.09.02

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

src/language/en-GB/tpl_mokocassiopeia.ini

@@ -238,6 +238,11 @@ TPL_MOKOCASSIOPEIA_CSS_VARS_VM_DESC="<strong>Surfaces &amp; text</strong><br><co
 TPL_MOKOCASSIOPEIA_CSS_VARS_GABLE_LABEL="Gable"
 TPL_MOKOCASSIOPEIA_CSS_VARS_GABLE_DESC="Colour tokens used by the Gable extension.<br><code>--gab-blue</code> — <code>#0066cc</code><br><code>--gab-green</code> — <code>#28a745</code><br><code>--gab-red</code> — <code>#dc3545</code><br><code>--gab-orange</code> — <code>#fd7e14</code><br><code>--gab-gray1</code> — <code>#495057</code><br><code>--gab-gray2</code> — <code>#6c757d</code><br><code>--gab-gray3</code> — <code>#adb5bd</code>"
 
+; ===== Theme Preview tab =====
+TPL_MOKOCASSIOPEIA_THEME_PREVIEW_FIELDSET_LABEL="Theme Preview"
+TPL_MOKOCASSIOPEIA_THEME_PREVIEW_INTRO="<p>Live preview of all CSS variables, hero variants, block colours, and Bootstrap components rendered with your active theme. Use the <strong>Toggle Light / Dark</strong> button inside the preview to switch modes. This page is also available as a standalone file at <code>templates/mokocassiopeia/templates/theme-test.html</code>.</p>"
+TPL_MOKOCASSIOPEIA_THEME_PREVIEW_FRAME="<iframe src='../templates/mokocassiopeia/templates/theme-test.html' style='width:100%;height:80vh;border:1px solid #dee2e6;border-radius:.375rem;' loading='lazy' title='Theme test sheet preview'></iframe>"
+
 ; ===== Misc =====
 MOD_BREADCRUMBS_HERE="YOU ARE HERE:"