IPv6: STCP dual-stack + Android config + invite fixes

- stcp_server: dual-stack (AF_INET6 + IPV6_V6ONLY=0) instead of IPv4-only
- stcp_client: getaddrinfo(AF_UNSPEC) instead of inet_pton(AF_INET)
- stcp_link: detect address family from remote_addr->ss_family
- platform_compat.h: add <netdb.h> for getaddrinfo on POSIX
- ConfigProvider.kt: add [server:main6] addr=[::]:PORT for Android
- invite_link_c.c: invite_serialize_addrs handles IPv6 (was IPv4-only)

tools/chatgui-android/app/src/main/java/com/utun/chat/data/ConfigProvider.kt

@@ -290,6 +290,9 @@ class ConfigProvider(private val context: Context) {
             appendLine("[server:main]")
             appendLine("addr=0.0.0.0:$listenPort")
             appendLine()
+            appendLine("[server:main6]")
+            appendLine("addr=[::]:$listenPort")
+            appendLine()
             appendLine("[allowed_keys]")
             appendLine("allow_all=yes")
             appendLine()