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/components.css

@@ -1045,6 +1045,7 @@
   /* ── Shop header ── */
 
   .shop-header {
+    position: relative;
     background-color: var(--t-surface-raised);
     border-bottom: 1px solid var(--t-border-default);
     display: flex;