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.php

@@ -33,7 +33,7 @@ document.addEventListener('htmx:afterSwap',()=>{document.querySelectorAll('input
                 </div>
             </div>
             <div class="admin-btn-group">
-                <a href="/admin/add.php" class="btn btn--primary btn--sm">+ Ajouter un TFE</a>
+                <a href="/admin/add.php" class="btn btn--primary btn--sm"><?= icon('plus-circle') ?> Ajouter un TFE</a>
                 <?php if ($trashCount > 0): ?>
                 <a href="/admin/index.php?tab=trash" class="btn btn--sm <?= $tab === 'trash' ? 'btn--primary' : 'btn--secondary' ?>">
                     Corbeille (<?= $trashCount ?>)
@@ -42,12 +42,12 @@ document.addEventListener('htmx:afterSwap',()=>{document.querySelectorAll('input
                 <?php if ($tmpTotalCount > 0): ?>
                 <button type="button" class="btn btn--sm btn--secondary" id="tmp-cleanup-btn"
                         onclick="document.getElementById('tmp-cleanup-dialog').showModal(); htmx.trigger('#tmp-cleanup-stats-wrapper','loadStats'); htmx.trigger('#peertube-orphans-wrapper','loadPeertube')">
-                    Nettoyer (<?= $tmpTotalCount ?>)
+                    <?= icon('trash') ?> Nettoyer (<?= $tmpTotalCount ?>)
                 </button>
                 <?php endif; ?>
                 <button type="button" class="btn btn--primary btn--sm" id="import-dialog-btn"
                         onclick="document.getElementById('import-dialog').showModal(); window.XamxamInitFilePonds()">
-                    Importer
+                    <?= icon('tray-arrow-up') ?> Importer
                 </button>
             </div>
         </div>