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/public/admin/fragments/peertube-browser.php

@@ -98,7 +98,7 @@ if (empty($orphans)) {
                     <?= $v['linkedTo'] !== null ? 'disabled title="Déjà liée au TFE ' . htmlspecialchars($v['linkedTo']) . '"' : '' ?>
                     >
                 <span class="file-browser-icon">
-                    <img src="/assets/icons/arrow-circle-left.svg" width="24" height="24" alt="" aria-hidden="true">
+                    <?= icon('arrow-circle-left') ?>
                 </span>
                 <span class="file-browser-name"><?= htmlspecialchars($v['name']) ?></span>
                 <span class="file-browser-size"><?= !empty($v['createdAt']) ? substr($v['createdAt'], 0, 10) : '' ?></span>