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/markdown-cheatsheet-fragment.php

@@ -95,7 +95,7 @@ $rows = [
             onclick="this.closest('dialog').close()"
             title="Fermer"
             aria-label="Fermer">
-      <img src="/assets/icons/x-circle.svg" width="24" height="24" alt="" aria-hidden="true">
+      <?= icon('x-circle') ?>
     </button>
   </div>