feat(amneziawg): support persistent keepalive ranges (#6881)

AmneziaWG clients accept a randomised PersistentKeepalive range such as 15-25,
which makes keepalive timing harder to fingerprint; the panel only stored a
single number. A client's keepalive may now be a range, validated the way
AmneziaWG parses it. The downloadable .conf and vpn:// profile carry the range,
Clash/Mihomo output gets its lower bound, and plain WireGuard stays scalar-only.
The range lives in an additive wg_keep_alive_range column, so the integer
column and an older panel binary keep working after a rollback.

internal/web/translation/es-ES.json

@@ -983,6 +983,8 @@
       "amneziaWgForwardedPorts": "Puertos reenviados",
       "amneziaWgForwardedPortsHint": "Puertos/rangos redirigidos (DNAT) a este cliente, p. ej. 80, 443, 8000-8100. Déjalo vacío si no aplica.",
       "tunnelKeepAlive": "Keepalive (seconds)",
+      "amneziaWgKeepAliveHint": "Indica los segundos como un número o un intervalo (por ejemplo, 15-25). Los valores deben estar entre 0 y 65535; 0 desactiva el keepalive.",
+      "amneziaWgKeepAliveInvalid": "Introduce un número entero o un intervalo entre 0 y 65535.",
       "tunnelKeepAliveHint": "How often the client sends a keepalive packet. 25 is the usual value: it holds the NAT mapping open and brings an idle peer back on its own after any drop, including a panel restart. 0 disables it — an idle client then stays disconnected until it sends traffic.",
       "amneziaWgConfig": "Configuración de AmneziaWG",
       "tuicConfig": "Configuración de TUIC (Clash)",