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.

frontend/src/schemas/client.ts

@@ -1,4 +1,5 @@
 import { z } from 'zod';
+import { OptionalClientKeepAliveSchema } from '@/schemas/primitives/keepalive';
 
 const nullableStringArray = z
   .array(z.string())
@@ -52,7 +53,7 @@ export const ClientRecordSchema = z
     publicKey: z.string().optional(),
     allowedIPs: z.string().optional(),
     preSharedKey: z.string().optional(),
-    keepAlive: z.number().optional(),
+    keepAlive: OptionalClientKeepAliveSchema,
     forwardedPorts: z.string().optional(),
     secret: z.string().optional(),
     adTag: z.string().optional(),