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/zh-CN.json

@@ -983,6 +983,8 @@
       "amneziaWgForwardedPorts": "转发端口",
       "amneziaWgForwardedPortsHint": "转发到此客户端的端口/范围，例如 80, 443, 8000-8100。留空则不转发。",
       "tunnelKeepAlive": "Keepalive (seconds)",
+      "amneziaWgKeepAliveHint": "秒数可输入数字或范围（例如 15-25）。数值须在 0 到 65535 之间；0 表示禁用 keepalive。",
+      "amneziaWgKeepAliveInvalid": "请输入 0 到 65535 之间的整数或范围。",
       "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": "AmneziaWG 配置",
       "tuicConfig": "TUIC 配置 (Clash)",