feat(invoice): add Send invoice email action

Adds a Send button to the order action extension and a corresponding
"send" op to /api/orders/:orderId/invoice. Generates the invoice on
demand if missing, then sends via the configured SMTP.

extensions/invoice-order-action/src/ActionExtension.tsx

@@ -13,7 +13,7 @@ function Extension() {
   const orderGid: string | undefined = data?.selected?.[0]?.id;
   const orderId = orderGid ? orderGid.split("/").pop() : undefined;
 
-  async function trigger(action: "generate" | "cancel_reissue") {
+  async function trigger(action: "generate" | "cancel_reissue" | "send") {
     if (!orderId) {
       setError("No order selected");
       return;
@@ -47,6 +47,9 @@ function Extension() {
       <s-button slot="primary-action" onClick={() => trigger("generate")} disabled={busy}>
         Generate / regenerate
       </s-button>
+      <s-button slot="secondary-actions" onClick={() => trigger("send")} disabled={busy}>
+        Send invoice email
+      </s-button>
       <s-button slot="secondary-actions" tone="critical" onClick={() => trigger("cancel_reissue")} disabled={busy}>
         Cancel &amp; reissue
       </s-button>