fix: default to 0.0.0.0 to fix WSL2 health check failure (#520)

PR #470 changed the default listen host to undefined, letting Node.js
bind to IPv6 :: on systems that support it. This broke WSL2 where IPv6
dual-stack is unreliable — the server binds to :: but IPv4 127.0.0.1
connections fail, causing the health check to time out.

Revert to 0.0.0.0 as the default. Users who need IPv6 can set
BIND_HOST=:: explicitly.

Fixes #518

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

packages/server/src/config.ts

@@ -8,8 +8,7 @@ export function getListenHost(env: Record<string, string | undefined> = process.
 
 export const config = {
   port: parseInt(process.env.PORT || '8648', 10),
-  // Leave host undefined by default so Node binds to IPv6 when available,
-  // falling back to IPv4 on systems without IPv6 support.
+  // Default undefined: listenWithFallback tries :: first, falls back to 0.0.0.0
   host: getListenHost(),
   upstream: process.env.UPSTREAM || 'http://127.0.0.1:8642',
   uploadDir: process.env.UPLOAD_DIR || resolve(homedir(), '.hermes-web-ui', 'upload'),