feat(amneziawg): restore per-client public IPv6 identity (Phase 3.5)

Adds internal/amneziawg.FirstIPv6 and a new internal/amneziawgnet/v6alias.go
that aliases each IPv6-enabled peer's own address onto the host NIC
(ip -6 addr add), wired into the Manager's Ensure/Remove/Reconcile/StopAll
lifecycle. internal/web/service/xray.go's new injectAmneziawgV6Egress gives
each such peer a dedicated freedom outbound (sendThrough) plus a routing
rule matching its own email, so its outbound connections carry a distinct
public source address again -- restoring what the embedded-architecture
hard cutover temporarily dropped. Scoped to outbound source identity only
(not unsolicited inbound/port-forwarding, which stays the separate Phase
3.6); no frontend changes needed since IPv6Enabled/IPv6ExternalInterface
were already in the UI and per-peer opt-in is just an IPv6 AllowedIPs entry,
same as today.

internal/web/translation/es-ES.json

@@ -1648,12 +1648,12 @@
         "primaryDns": "DNS primario",
         "secondaryDns": "DNS secundario",
         "externalInterface": "Interfaz externa",
-        "externalInterfaceHint": "Interfaz de red del host para NAT (PostUp/PostDown). Déjalo vacío para autodetectar.",
+        "externalInterfaceHint": "Interfaz de red del host para el alias de la dirección IPv6 cuando se deja vacía la Interfaz Externa IPv6.",
         "ipv6Enabled": "Habilitar IPv6",
         "ipv6Subnet": "Subred IPv6",
         "ipv6SubnetHint": "p. ej. fd86:ea04:1115::/64. Obligatorio cuando IPv6 está habilitado.",
         "ipv6ExternalInterface": "Interfaz externa IPv6",
-        "ipv6ExternalInterfaceHint": "Interfaz de red del host para las entradas de proxy NDP. Déjalo vacío para reutilizar la interfaz externa.",
+        "ipv6ExternalInterfaceHint": "Interfaz de red del host en la que se alía la dirección IPv6 de cada cliente. Déjalo vacío para reutilizar la Interfaz Externa.",
         "obfuscation": "Parámetros de ofuscación",
         "regenerateObfuscation": "Regenerar",
         "jc": "Jc (cantidad de paquetes basura)",