feat(server): add LISTEN_ADDR env var for allowing server to listen on ipv6 (#14211)

The old code hardcoded 0.0.0.0 which means the server only listened for
ipv4 connections, making it not work on ipv6-only networks.

This change adds a LISTEN_ADDR env var which allows the server to bind
to ipv6 as well.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Server listen address is now configurable via the LISTEN_ADDR
environment variable (default: 0.0.0.0), enabling IPv4/IPv6 or
interface-specific binding.
* Configuration schemas and admin UI now expose the listen address
option so deployments can view and override it.

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settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/admin/src/config.json

@@ -216,6 +216,11 @@
       "type": "Array",
       "desc": "Multiple hosts the server will accept requests from."
     },
+    "listenAddr": {
+      "type": "String",
+      "desc": "The address to listen on (e.g., 0.0.0.0 for IPv4, :: for IPv6).",
+      "env": "LISTEN_ADDR"
+    },
     "port": {
       "type": "Number",
       "desc": "Which port the server will listen on.",