fix: duplicate warning not shown in admin, double-encoded in partage, no focus

- toast-fragment.php: 204 early-exit now also checks flash['warning'];
  previously the warning was consumed by consumeFlash() then silently dropped
- partage/index.php: store warning as plain text; htmlspecialchars() applied
  once at render time — previously htmlspecialchars() was called inside the
  stored string then again at output, producing ' entities etc.
- partage/index.php: flash-warning div gets id + tabindex=-1; inline JS
  scrolls it into view and focuses it on DOMContentLoaded
- admin/footer.php: htmx:afterSettle listener focuses .toast--warning after
  HTMX injects the toast fragment into #toast-region

app/templates/admin/footer.php

@@ -14,5 +14,13 @@
 <script><?= $extraJsInline ?></script>
 <?php endif; ?>
 <script src="/assets/js/htmx.min.js"></script>
+<script>
+document.body.addEventListener('htmx:afterSettle', function (e) {
+    if (e.target && e.target.id === 'toast-region') {
+        var warn = e.target.querySelector('.toast--warning');
+        if (warn) { warn.setAttribute('tabindex', '-1'); warn.focus(); }
+    }
+});
+</script>
 </body>
 </html>