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/email_settings_test.exs

@@ -82,11 +82,11 @@ defmodule BerrypodWeb.Admin.EmailSettingsTest do
       |> form("form[phx-change=\"change_adapter\"]", %{email: %{adapter: "postmark"}})
       |> render_change()
 
-      # Submit with an API key
+      # Submit with an API key (Postmark uses UUID format)
       html =
         view
         |> form("form[phx-submit=\"save\"]", %{
-          email: %{adapter: "postmark", api_key: "pm_test_123"}
+          email: %{adapter: "postmark", api_key: "abc12345-abcd-1234-abcd-123456789abc"}
         })
         |> render_submit()
 
@@ -108,7 +108,7 @@ defmodule BerrypodWeb.Admin.EmailSettingsTest do
         |> form("form[phx-submit=\"save\"]", %{email: %{adapter: "postmark", api_key: ""}})
         |> render_submit()
 
-      assert html =~ "Missing required fields"
+      assert html =~ "API key is required"
     end
 
     test "disconnect clears email configuration", %{conn: conn} do
@@ -167,7 +167,7 @@ defmodule BerrypodWeb.Admin.EmailSettingsTest do
 
       view
       |> form("form[phx-submit=\"save\"]", %{
-        email: %{adapter: "postmark", api_key: "pm_new_key"}
+        email: %{adapter: "postmark", api_key: "def12345-abcd-1234-abcd-123456789def"}
       })
       |> render_submit()