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/partials/dialogs/tmp-cleanup.php

@@ -15,7 +15,7 @@
                  hx-indicator="#tmp-cleanup-stats-wrapper">
                 <details class="n-section" open>
                     <summary>
-                        <img src="/assets/icons/paint-brush-household.svg" width="14" height="14" alt="" aria-hidden="true">
+                        <?= icon('paint-brush-household') ?>
                         Fichiers temporaires
                     </summary>
                     <p style="margin:0;color:var(--text-secondary)">Chargement…</p>