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/public/about.php

@@ -36,7 +36,7 @@ function renderEntries(array $entries): string
     <!-- Table of contents: collapsible on mobile, force-open on desktop -->
     <details class="toc" open>
         <summary><?= icon('caret-down', 0, 'toc-caret') ?> PARTIES</summary>
-        <ul>
+        <ul class="toc-list">
             <li><a href="#apropos-intro">À propos</a></li>
             <?php if (!empty($contacts)): ?>
             <li><a href="#apropos-contacts">Contacts</a></li>
@@ -54,6 +54,7 @@ function renderEntries(array $entries): string
         <?php endif; ?>
     </details>
 
+    <article>
     <!-- Intro text from DB -->
     <section class="content-section" id="apropos-intro">
         <?= $aboutHtml ?>
@@ -113,5 +114,6 @@ function renderEntries(array $entries): string
                 </div>
             </dl>
     </section>
+    </article>
 
 </main>