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/search-bar.php

@@ -6,7 +6,7 @@ $_sbValue = $searchBarValue ?? $_GET['query'] ?? '';
 <form method="GET" action="/search"
       role="search" aria-label="Recherche">
     <label for="site-search-input" class="sr-only">Recherche</label>
-    <img src="/assets/icons/search.svg" width="24" height="24" alt="" aria-hidden="true">
+    <?= icon('search') ?>
     <input
         id="site-search-input"
         type="text"