improve notification accessibility

- use role="status" for info messages, role="alert" for errors
- add aria-live attribute (polite for info, assertive for errors)
- move phx-click to close button for better keyboard navigation
- add close buttons to shop flash messages
- add aria-hidden to decorative icons

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/css/shop/components.css

@@ -2491,6 +2491,22 @@
     color: hsl(0 70% 50%);
   }
 
+  .shop-flash-close {
+    margin-left: auto;
+    padding: 0.25rem;
+    cursor: pointer;
+    opacity: 0.5;
+    transition: opacity 150ms ease;
+    border-radius: 0.25rem;
+
+    &:hover { opacity: 0.8; }
+    &:focus-visible {
+      opacity: 1;
+      outline: 2px solid var(--t-accent);
+      outline-offset: 2px;
+    }
+  }
+
   /* Transition classes for JS.hide flash dismiss */
   .fade-out { transition: opacity 200ms ease-out; }
   .fade-out-from { opacity: 1; }