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/uk-UA.json

@@ -983,6 +983,8 @@
       "amneziaWgForwardedPorts": "Перенаправлені порти",
       "amneziaWgForwardedPortsHint": "Порти/діапазони, що перенаправляються (DNAT) на цього клієнта, напр. 80, 443, 8000-8100. Залиште порожнім, якщо не потрібно.",
       "tunnelKeepAlive": "Keepalive (seconds)",
+      "amneziaWgKeepAliveHint": "Укажіть секунди числом або діапазоном (наприклад, 15-25). Значення має бути від 0 до 65535; 0 вимикає keepalive.",
+      "amneziaWgKeepAliveInvalid": "Введіть ціле число або діапазон від 0 до 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": "Конфігурація AmneziaWG",
       "tuicConfig": "Конфігурація TUIC (Clash)",