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>

test/berrypod_web/live/admin/providers_test.exs

@@ -174,11 +174,14 @@ defmodule BerrypodWeb.Admin.ProvidersTest do
 
       {:ok, view, _html} = live(conn, ~p"/admin/providers/new")
 
+      # Printify uses 36-character UUID format keys
+      valid_uuid_key = "12345678-1234-1234-1234-123456789012"
+
       {:ok, _view, html} =
         view
         |> form("#provider-form", %{
           "provider_connection" => %{
-            "api_key" => "test_key_123"
+            "api_key" => valid_uuid_key
           }
         })
         |> render_submit()