Fix biome lint errors: remove duplicate CSS properties, apply safe auto-fixes

CSS:
- Remove duplicate 'background' fallbacks in base.css, header.css, search.css
  (solid color declared before gradient — gradient always wins)
- Remove duplicate 'padding' in admin.css .admin-import-log

JS (biome --write safe fixes applied):
- function() → arrow functions in all IIFEs and callbacks
- forEach/callback → arrow functions
- evaluePtrn → parseInt(x, 10) in admin-contacts-form.js
- Cleaned label text in build.mjs lint step

Remaining warnings are intentional: !important overrides, descending
specificity (admin.css cascade), noUnusedVariables (functions exported
to window/onclick), useTemplate style preference.

app/templates/admin/partials/admin-toc.php

@@ -2,12 +2,14 @@
 /**
  * admin-toc.php — sidebar table-of-contents for long admin pages.
  *
- * Rendered as an <aside> inside <main>, before the <article> content.
- * Uses IntersectionObserver to highlight the active section.
+ * Uses the same <details class="toc"> markup as the public content pages
+ * (about, charte, licence). The link list is populated by admin-toc.js
+ * at runtime.
  */
 ?>
-<aside id="admin-toc" class="admin-toc" aria-label="Sur cette page">
-    <nav class="admin-toc-list" id="admin-toc-list">
-        <!-- populated by JS (admin-toc.js, loaded via admin.min.js) -->
-    </nav>
-</aside>
+<details class="toc" id="admin-toc" open aria-label="Sur cette page">
+    <summary><?= icon('caret-down', 0, 'toc-caret') ?> SUR CETTE PAGE</summary>
+    <ul class="toc-list" id="admin-toc-list">
+        <!-- populated by admin-toc.js -->
+    </ul>
+</details>