fix: use real MagicDNS hostname in serve.headscale.json (focalboard.damconsulting.net)

The {{service}}.yourdomain.com placeholder does not work in tailscale serve -
JSON values are not template-substituted. Use the actual MagicDNS base
domain so the user has a working starting point, matching the pattern
established in ts-homepage.

tailscale/config/serve.headscale.json

@@ -5,7 +5,7 @@
     }
   },
   "Web": {
-    "{{service}}.yourdomain.com:80": {
+    "focalboard.damconsulting.net:80": {
       "Handlers": {
         "/": {
           "Proxy": "http://127.0.0.1:8000"
@@ -14,4 +14,3 @@
     }
   }
 }
-