Take the upstream name out of the mock's sample data

The mock inbox showed a sender called "Stalwart Labs" at [email protected],
a "Welcome to Stalwart!" subject, a link to stalw.art in the sample HTML
and a start-up banner tagged [mock-stalwart]. None of that belongs in this
fork, and it turns up in any screenshot taken from the mock.

Sample senders and subjects now name inbuxa, the sample link points at
inbuxa.org, and the banner says [mock-server].

server/src/mock/index.ts

@@ -204,8 +204,8 @@ export const server = createServer(async (req, res) => {
   res.writeHead(404, { "content-type": "application/json" });
   res.end(JSON.stringify({ error: "not found" }));
 }).listen(PORT, "127.0.0.1", () => {
-  console.log(`[mock-stalwart] listening on http://127.0.0.1:${PORT}  (login: ${USER} / ${PASS})`);
-  console.log(`[mock-stalwart] run the app with: MAIL_SERVER_URL=http://127.0.0.1:${PORT} npm run dev`);
+  console.log(`[mock-server] listening on http://127.0.0.1:${PORT}  (login: ${USER} / ${PASS})`);
+  console.log(`[mock-server] run the app with: MAIL_SERVER_URL=http://127.0.0.1:${PORT} npm run dev`);
 });
 
 // Periodically inject a new inbox email to demo push