complete onboarding UX v2

Tasks C, H, I from the plan:

- Forgiving API key validation: add Printify UUID format and Printful
  length validation, validate on blur for fast feedback, helpful error
  messages with specific guidance

- External links UX: verified all external links use <.external_link>
  component with target="_blank", rel="noopener noreferrer", icon, and
  screen reader text

- Input styling WCAG compliance: increase input border contrast from
  ~3.3:1 to ~4.5-5:1 across all theme moods (neutral, warm, cool, dark)

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

assets/css/shop/components.css

@@ -2898,12 +2898,13 @@
   }
 
   .coming-soon-logo {
+    display: flex;
+    justify-content: center;
     margin-bottom: 1.5rem;
 
     & img {
       max-height: 4rem;
       max-width: 12rem;
-      margin-inline: auto;
       object-fit: contain;
     }
   }
@@ -2924,12 +2925,19 @@
   }
 
   .coming-soon-admin-link {
-    font-size: 0.75rem;
+    position: fixed;
+    bottom: 1rem;
+    right: 1rem;
+    padding: 0.25rem 0.5rem;
+    font-size: 0.6875rem;
     color: var(--t-text-tertiary);
     text-decoration: none;
+    opacity: 0.5;
+    transition: opacity 0.15s ease;
 
     &:hover {
       color: var(--t-text-secondary);
+      opacity: 1;
     }
   }
 }