Fix nil-pointer crash generating QR codes for non-default servers

GetClients, GetClientByID, and SendRequestedConfigsToTelegram all
looked up the legacy "wg0" server unconditionally when building a
client's QR code / config, ignoring which server the client actually
belongs to. On any multi-server setup without a migrated wg0, the
server lookup returned a zero-value model.Server (error discarded),
and BuildClientConfig then dereferenced its nil KeyPair pointer,
panicking whenever a client's QR code was rendered.

Now looks up the client's own ServerID (falling back to wg0 only for
legacy clients with no ServerID set) and surfaces the lookup error
instead of silently continuing with an empty server.

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

util/util.go

@@ -633,7 +633,15 @@ func SendRequestedConfigsToTelegram(db store.IStore, userid int64) []string {
 			}
 
 			// build config
-			server, _ := db.GetServerByID(DefaultServerID)
+			serverID := clientData.Client.ServerID
+			if serverID == "" {
+				serverID = DefaultServerID
+			}
+			server, err := db.GetServerByID(serverID)
+			if err != nil {
+				failedList = append(failedList, clientData.Client.Name)
+				continue
+			}
 			globalSettings, _ := db.GetGlobalSettings()
 			config := BuildClientConfig(*clientData.Client, server, globalSettings)
 			configData := []byte(config)