Fix contact form: use host networking to reach Proton Bridge SMTP

Both containers now run in network_mode: host so the API can connect
directly to Proton Bridge on 127.0.0.1:1025. The pfSense search domain
(home.arpa) was leaking into Docker DNS and causing NXDOMAIN failures
for inter-container hostnames. Host networking bypasses this entirely.

- docker-compose: both services use network_mode: host
- nginx: listen on 8080 (was 80), proxy /api/ to 127.0.0.1:3001
- server.js: allow self-signed TLS cert from Proton Bridge

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

backend/server.js

@@ -14,6 +14,7 @@ const transporter = nodemailer.createTransport({
     user: process.env.SMTP_USER,
     pass: process.env.SMTP_PASS,
   },
+  tls: { rejectUnauthorized: false },
 })
 
 app.post('/contact', async (req, res) => {