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>

tests/Architecture/FrontendRuntimeHostContractTest.php

@@ -12,7 +12,7 @@ class FrontendRuntimeHostContractTest extends TestCase
     {
         $defaultTemplate = $this->readProjectFile('templates/default.phtml');
         $loginTemplate = $this->readProjectFile('templates/login.phtml');
-        $flashPartial = $this->readProjectFile('templates/partials/app-flash.phtml');
+        $flashPartial = $this->readProjectFile('templates/partials/app-toast-stack.phtml');
         $topbarPartial = $this->readProjectFile('templates/partials/app-topbar.phtml');
         $asideIconBar = $this->readProjectFile('templates/partials/app-main-aside-icon-bar.phtml');
         $asidePartial = $this->readProjectFile('templates/partials/app-main-aside.phtml');