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/apropos-groups-form.php

@@ -51,7 +51,7 @@
                 data-key="<?= $aproposKey ?>">+ Ajouter une entrée</button>
         <button type="button" class="admin-icon-btn admin-icon-btn--delete delete-group-btn-f"
                 data-key="<?= $aproposKey ?>" title="Supprimer ce contact">
-            <img src="/assets/icons/trash-slash.svg" width="20" height="20" alt="" aria-hidden="true">
+            <?= icon('trash-slash') ?>
         </button>
     </fieldset>
     <?php endforeach; ?>
@@ -88,7 +88,7 @@
                     data-key="<?= $aproposKey ?>">+ Ajouter une entrée</button>
             <button type="button" class="admin-icon-btn admin-icon-btn--delete delete-group-btn-f"
                     data-key="<?= $aproposKey ?>" title="Supprimer ce contact">
-                <img src="/assets/icons/trash-slash.svg" width="20" height="20" alt="" aria-hidden="true">
+                <?= icon('trash-slash') ?>
             </button>
         </fieldset>
     </template>