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/partials/form/language-search.php

@@ -45,7 +45,7 @@ $langCount      = count($selectedLanguages);
                 <input type="hidden" name="<?= htmlspecialchars($name) ?>[]" value="<?= htmlspecialchars($lang['name']) ?>">
                 <span class="tag-pill-name"><?= htmlspecialchars($lang['name']) ?></span>
                 <button type="button" class="tag-pill-remove" title="Retirer «&nbsp;<?= htmlspecialchars($lang['name']) ?>&nbsp;»" aria-label="Retirer <?= htmlspecialchars($lang['name']) ?>">
-                    <img src="/assets/icons/trash-slash.svg" width="16" height="16" alt="" aria-hidden="true">
+                    <?= icon('trash-slash') ?>
                 </button>
             </span>
             <?php endforeach; ?>