add admin email settings page with provider selection

Card radio component for picking email providers (SMTP, SendGrid, Mailjet, etc.)
with instant client-side switching via JS hook. Adapter configs are pre-rendered
and toggled without a server round-trip. Secrets are preserved when re-saving
with blank password fields. Includes from address field, test email sending,
and disconnect flow.

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

lib/berrypod_web/components/layouts/admin.html.heex

@@ -23,6 +23,9 @@
       <.icon name="hero-exclamation-triangle" class="size-5 shrink-0" />
       <p>
         Email delivery isn't set up yet — customers won't receive order confirmations or shipping updates.
+        <.link navigate={~p"/admin/settings/email"} class="underline font-medium">
+          Configure email
+        </.link>
       </p>
     </div>
 
@@ -99,6 +102,14 @@
               <.icon name="hero-cog-6-tooth" class="size-5" /> Settings
             </.link>
           </li>
+          <li>
+            <.link
+              navigate={~p"/admin/settings/email"}
+              class={admin_nav_active?(@current_path, "/admin/settings/email")}
+            >
+              <.icon name="hero-envelope" class="size-5" /> Email
+            </.link>
+          </li>
         </ul>
       </nav>