add header background contrast warning and improve branding UX

- extract dominant colors from header images during optimization
- calculate WCAG contrast ratios against theme text color
- show warning in theme editor when text may be hard to read
- prevent hiding shop name when no logo is uploaded
- auto-enable shop name when logo is deleted
- fix image cache invalidation on delete
- add missing .hidden utility class

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

assets/css/shop/utilities.css

@@ -32,6 +32,10 @@
     text-wrap: balance;
   }
 
+  .hidden {
+    display: none;
+  }
+
   /* Hide visually but keep in DOM (for phx-update="stream" empty states etc.) */
   .visually-hidden:not(:focus):not(:active) {
     position: absolute;