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.

app/routes/api.orders.$orderId.invoice.tsx

@@ -3,6 +3,7 @@ import { authenticate } from "../shopify.server";
 import db from "../db.server";
 import { generateInvoice } from "../services/invoice/generateInvoice.server";
 import { cancelAndReissue } from "../services/invoice/cancelAndReissue.server";
+import { sendInvoiceEmail } from "../services/invoice/email.server";
 
 /**
  * GET  /api/orders/:orderId/invoice  → returns latest invoice metadata + history
@@ -60,6 +61,50 @@ export const action = async ({ request, params }: ActionFunctionArgs) => {
       return cors(Response.json({ ok: true, op, ...result }));
     }
 
+    if (op === "send") {
+      const orderGid = orderId.startsWith("gid://")
+        ? orderId
+        : `gid://shopify/Order/${orderId}`;
+      let invoice = await db.invoice.findFirst({
+        where: {
+          shopDomain: session.shop,
+          orderId: orderGid,
+          kind: "invoice",
+          cancelledAt: null,
+        },
+        orderBy: [{ version: "desc" }, { createdAt: "desc" }],
+      });
+      if (!invoice) {
+        const generated = await generateInvoice({
+          shopDomain: session.shop,
+          admin,
+          orderId,
+        });
+        invoice = await db.invoice.findUnique({ where: { id: generated.invoiceId } });
+      }
+      if (!invoice) throw new Error("Failed to materialise an invoice for this order.");
+      const sendResult = await sendInvoiceEmail({
+        shopDomain: session.shop,
+        invoiceId: invoice.id,
+      });
+      if (!sendResult.ok) {
+        return cors(
+          Response.json(
+            { ok: false, op: "send", error: sendResult.errorMessage ?? "Email send failed." },
+            { status: 422 },
+          ),
+        );
+      }
+      return cors(
+        Response.json({
+          ok: true,
+          op: "send",
+          invoiceNumber: invoice.invoiceNumber,
+          toAddress: sendResult.toAddress,
+        }),
+      );
+    }
+
     const result = await generateInvoice({
       shopDomain: session.shop,
       admin,