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/index-trash.php

@@ -2,7 +2,7 @@
     <div class="admin-list-toolbar admin-list-toolbar--list">
         <div class="admin-toolbar-top">
             <div class="admin-toolbar-title-row">
-                <h1><a href="/admin/" class="admin-back-btn" title="Retour à la liste"><img src="/assets/icons/arrow-left-circle.svg" width="32" height="32" alt="" aria-hidden="true"></a> Corbeille</h1>
+                <h1><a href="/admin/" class="admin-back-btn" title="Retour à la liste"><?= icon('arrow-left-circle') ?></a> Corbeille</h1>
                 <span class="admin-stat admin-stat--inline" style="margin-left:auto"><?= count($trashedTheses) ?> TFE(s)</span>
             </div>
         </div>
@@ -60,7 +60,7 @@
                                 <input type="hidden" name="action" value="restore">
                                 <input type="hidden" name="thesis_id" value="<?= (int)$t['id'] ?>">
                                 <button type="submit" class="admin-icon-btn admin-icon-btn--edit" title="Restaurer">
-                                    <img src="/assets/icons/archive-box.svg" width="32" height="32" alt="" aria-hidden="true">
+                                    <?= icon('archive-box') ?>
                                 </button>
                             </form>
                             <form method="post" action="actions/corbeille.php" class="admin-inline-form" style="display:inline"
@@ -69,7 +69,7 @@
                                 <input type="hidden" name="action" value="hard_delete">
                                 <input type="hidden" name="thesis_id" value="<?= (int)$t['id'] ?>">
                                 <button type="submit" class="admin-icon-btn admin-icon-btn--delete" title="Supprimer définitivement">
-                                    <img src="/assets/icons/trash.svg" width="32" height="32" alt="" aria-hidden="true">
+                                    <?= icon('trash') ?>
                                 </button>
                             </form>
                         </div>