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>

store/jsondb/jsondb.go

@@ -412,7 +412,14 @@ func (o *JsonDB) GetClients(hasQRCode bool) ([]model.ClientData, error) {
 
 		// generate client qrcode image in base64
 		if hasQRCode && client.PrivateKey != "" {
-			server, _ := o.GetServerByID(util.DefaultServerID)
+			serverID := client.ServerID
+			if serverID == "" {
+				serverID = util.DefaultServerID
+			}
+			server, err := o.GetServerByID(serverID)
+			if err != nil {
+				fmt.Printf("Cannot generate QR code: server %s not found: %v\n", serverID, err)
+			} else {
 				globalSettings, _ := o.GetGlobalSettings()
 
 				png, err := qrcode.Encode(util.BuildClientConfig(client, server, globalSettings), qrcode.Medium, 256)
@@ -422,6 +429,7 @@ func (o *JsonDB) GetClients(hasQRCode bool) ([]model.ClientData, error) {
 					fmt.Print("Cannot generate QR code: ", err)
 				}
 			}
+		}
 
 		// create the list of clients and their qrcode data
 		clientData.Client = &client
@@ -442,7 +450,14 @@ func (o *JsonDB) GetClientByID(clientID string, qrCodeSettings model.QRCodeSetti
 
 	// generate client qrcode image in base64
 	if qrCodeSettings.Enabled && client.PrivateKey != "" {
-		server, _ := o.GetServerByID(util.DefaultServerID)
+		serverID := client.ServerID
+		if serverID == "" {
+			serverID = util.DefaultServerID
+		}
+		server, err := o.GetServerByID(serverID)
+		if err != nil {
+			fmt.Printf("Cannot generate QR code: server %s not found: %v\n", serverID, err)
+		} else {
 			globalSettings, _ := o.GetGlobalSettings()
 			client := client
 			if !qrCodeSettings.IncludeDNS {
@@ -459,6 +474,7 @@ func (o *JsonDB) GetClientByID(clientID string, qrCodeSettings model.QRCodeSetti
 				fmt.Print("Cannot generate QR code: ", err)
 			}
 		}
+	}
 
 	clientData.Client = &client
 

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)