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/public/assets/css/admin.css

@@ -186,6 +186,7 @@
     background: var(--bg-secondary);
     border-color: var(--warning);
     color: var(--text-primary);
+    animation: toast-enter 0.35s ease-out; /* no fade-out — stays until dismissed */
 }
 
 .toast--warning a {