Auto-dismiss sync toasts after a few seconds

Fixes two issues found during manual verification:
- fragments/toast.html was missing class="toast-container" on the
  swapped-in element, so the container lost its fixed-position
  styling after the first swap.
- htmx:oobAfterSwap's event.detail.target is the *old* element that
  just got replaced (outerHTML oob-swaps detach it), so the dismiss
  timer must look up the live #toast-container by id instead of
  trusting that stale reference.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/web/templates/fragments/toast.html

@@ -1,3 +1,3 @@
-<div id="toast-container" hx-swap-oob="true">
+<div id="toast-container" class="toast-container" hx-swap-oob="true">
     <div class="toast toast-{{ level }}">{{ message }}</div>
 </div>