Migrate all <img>-based icons to inline SVG via PHP helper

Replace every <img src="/assets/icons/..."> with <?= icon('name') ?>
across 26 template files. The PHP helper inlines the SVG markup into the
DOM so CSS color cascades naturally through fill="currentColor".

- Add src/icon.php helper: reads SVG file, sets width/height to 1em,
  injects aria-hidden, supports optional CSS class
- Fix 12 icon SVGs that had hardcoded fill="#000000" or missing fill attr
- Replace search.svg with Phosphor fill-based magnifying glass
- Add explicit SVG sizes for admin header nav icons (16px/20px)
- Scope public search icon CSS to form[role=search]:not(.header-search-form)
  to avoid breaking admin header layout; change stroke to fill
- Remove <img> filter: brightness(0) invert(1) hacks from admin.css

app/templates/admin/contenus-edit.php

@@ -1,5 +1,5 @@
 <main id="main-content" class="full-editor-page">
-    <h1><a href="/admin/contenus.php" class="admin-back-btn" title="Retour"><img src="/assets/icons/arrow-left-circle.svg" width="32" height="32" alt="" aria-hidden="true"></a> Éditer : <?= htmlspecialchars($editTitle) ?></h1>
+    <h1><a href="/admin/contenus.php" class="admin-back-btn" title="Retour"><?= icon('arrow-left-circle') ?></a> Éditer : <?= htmlspecialchars($editTitle) ?></h1>
 
     <?php if ($editType === 'about_page'): ?>
 
@@ -63,7 +63,7 @@
             </div>
             <button type="button" class="admin-icon-btn admin-icon-btn--delete remove-sidebar-link-btn"
                     title="Supprimer ce lien">
-                <img src="/assets/icons/trash-slash.svg" width="20" height="20" alt="" aria-hidden="true">
+                <?= icon('trash-slash') ?>
             </button>
         </div>
         <?php endforeach; ?>
@@ -87,7 +87,7 @@
             </div>
             <button type="button" class="admin-icon-btn admin-icon-btn--delete remove-sidebar-link-btn"
                     title="Supprimer ce lien">
-                <img src="/assets/icons/trash-slash.svg" width="20" height="20" alt="" aria-hidden="true">
+                <?= icon('trash-slash') ?>
             </button>
         </div>
     </template>