refactor(ui): unify notice + toast styling, polish inline notices

Notices used to be styled only inside the toast stack — every inline
.notice on the login page, auth pages and admin edit screens fell back
to browser defaults and looked unstyled. The Stripe-style toast redesign
(icon pill + neutral text + soft card surface) now lives on the base
.notice rule, and .app-toast-stack adds the slide-in animation, soft
shadow, dismiss button and progress bar on top.

Inline notices auto-render the variant icon via a ::before pseudo using
the Bootstrap Icons codepoints, so all 30+ existing call sites get the
new look without markup changes. Toasts opt out of ::before via
:has(> .notice-icon) and keep their explicit icon span.

The previous app-flash.phtml partial is folded into app-toast-stack.phtml
(both create the same .app-toast-stack container — having both mounted
made two stacks fight for the same fixed corner). default/login/page
templates now mount the unified partial; the architecture contract is
updated to match.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

templates/page.phtml

@@ -53,7 +53,7 @@ if ($bufferTitle !== '') {
     </main>
     <?php require __DIR__ . '/partials/app-footer.phtml'; ?>
   </div>
-  <?php require __DIR__ . '/partials/app-flash.phtml'; ?>
+  <?php require __DIR__ . '/partials/app-toast-stack.phtml'; ?>
 </body>
 
 </html>