make admin provider UI support both Printify and Printful

- Provider form accepts ?type= query param (printify/printful)
- Conditional setup instructions per provider (API key steps, login URLs)
- Dynamic labels, titles, and config handling (shop_id vs store_id)
- Provider index shows dropdown with both provider options
- Settings page renamed from @printify to @provider (generic)
- Fix Printful shipping rates: add default state codes for US/CA/AU

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme/providers/printful.ex

@@ -198,8 +198,9 @@ defmodule SimpleshopTheme.Providers.Printful do
 
   defp fetch_rate_for_product(catalog_product_id, variant_id, country_code) do
     items = [%{source: "catalog", catalog_variant_id: variant_id, quantity: 1}]
+    recipient = build_recipient(country_code)
 
-    case Client.calculate_shipping(%{country_code: country_code}, items) do
+    case Client.calculate_shipping(recipient, items) do
       {:ok, rates} when is_list(rates) ->
         standard = Enum.find(rates, &(&1["shipping"] == "STANDARD")) || List.first(rates)
 
@@ -228,6 +229,18 @@ defmodule SimpleshopTheme.Providers.Printful do
     end
   end
 
+  # Printful requires state_code for US, CA, and AU
+  @default_state_codes %{"US" => "NY", "CA" => "ON", "AU" => "NSW"}
+
+  defp build_recipient(country_code) do
+    base = %{country_code: country_code}
+
+    case @default_state_codes[country_code] do
+      nil -> base
+      state -> Map.put(base, :state_code, state)
+    end
+  end
+
   # Returns {catalog_product_id, first_catalog_variant_id} per product
   defp extract_per_product_items(products) do
     products