add forgiving API key validation with inline errors

Add KeyValidation module for format-checking API keys before
attempting connections. Auto-strips whitespace, detects common
mistakes (e.g. pasting a Stripe publishable key), and returns
helpful error messages.

Inline field errors across all three entry points:
- Setup wizard: provider + Stripe keys
- Admin provider form: simplified to single Connect button
- Email settings: per-field errors instead of flash toasts

Also: plain text inputs for all API keys (not password fields),
accessible error states (aria-invalid, role=alert, thick border,
bold text), inner_block slot declaration on error component.

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

test/berrypod_web/live/admin/providers_test.exs

@@ -167,14 +167,6 @@ defmodule BerrypodWeb.Admin.ProvidersTest do
       assert html =~ "Connect to Printify"
     end
 
-    test "test connection shows error when no api key", %{conn: conn} do
-      {:ok, view, _html} = live(conn, ~p"/admin/providers/new")
-
-      html = render_click(view, "test_connection")
-
-      assert html =~ "Please enter your API key"
-    end
-
     test "saves new connection", %{conn: conn} do
       expect(MockProvider, :test_connection, fn _conn ->
         {:ok, %{shop_name: "My Printify Shop", shop_id: 12345}}
@@ -196,55 +188,6 @@ defmodule BerrypodWeb.Admin.ProvidersTest do
     end
   end
 
-  describe "form - test connection" do
-    setup %{conn: conn, user: user} do
-      Application.put_env(:berrypod, :provider_modules, %{
-        "printify" => MockProvider
-      })
-
-      on_exit(fn -> Application.delete_env(:berrypod, :provider_modules) end)
-
-      %{conn: log_in_user(conn, user)}
-    end
-
-    test "shows success when connection is valid", %{conn: conn} do
-      expect(MockProvider, :test_connection, fn _conn ->
-        {:ok, %{shop_name: "My Printify Shop", shop_id: 12345}}
-      end)
-
-      {:ok, view, _html} = live(conn, ~p"/admin/providers/new")
-
-      # Validate first to set pending_api_key
-      view
-      |> form("#provider-form", %{
-        "provider_connection" => %{"api_key" => "valid_key_123"}
-      })
-      |> render_change()
-
-      html = render_click(view, "test_connection")
-
-      assert html =~ "Connected to My Printify Shop"
-    end
-
-    test "shows error when connection fails", %{conn: conn} do
-      expect(MockProvider, :test_connection, fn _conn ->
-        {:error, :unauthorized}
-      end)
-
-      {:ok, view, _html} = live(conn, ~p"/admin/providers/new")
-
-      view
-      |> form("#provider-form", %{
-        "provider_connection" => %{"api_key" => "bad_key"}
-      })
-      |> render_change()
-
-      html = render_click(view, "test_connection")
-
-      assert html =~ "doesn't seem to be valid"
-    end
-  end
-
   describe "form - edit" do
     setup %{conn: conn, user: user} do
       connection =