Fix "no such file" error creating clients on non-default servers

SuggestIPAllocation always looked up the legacy "wg0" server regardless
of which server the request was for, so any setup without a migrated
wg0 (i.e. every fresh multi-server install) failed with
"open db/servers/wg0.json: no such file or directory" when adding a
new client.

Now accepts an optional server_id query param (falling back to wg0 for
the legacy bare routes), and the per-server clients page passes its
own server ID.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/server_clients.html

@@ -322,7 +322,7 @@ Wireguard Clients
             $.ajax({
                 cache: false,
                 method: 'GET',
-                url: `{{.basePath}}/api/suggest-client-ips?sr=${subnetRange}`,
+                url: `{{.basePath}}/api/suggest-client-ips?sr=${subnetRange}&server_id={{.serverID}}`,
                 dataType: 'json',
                 contentType: "application/json",
                 success: function(data) {