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>

lib/berrypod_web/live/admin/providers/form.html.heex

@@ -51,6 +51,7 @@
           else: "Paste your key here"
       }
       autocomplete="off"
+      phx-debounce="blur"
     />
 
     <%= if @live_action == :edit do %>