Add per-slave ESP-NOW OTA progress over UART and fix dashboard updates.

Expose OTA_SLAVE_PROGRESS on the master, track per-slave state during
distribution, run ESP-NOW OTA in a background task so the host can poll
while slaves update, and show master/slave progress in the dashboard
with table layout and faster WebSocket refresh during uploads.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/serial_link.go

@@ -1,12 +1,16 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"sync"
 	"time"
 )
 
+// errUARTBusy is returned when the port is held for OTA (poller should not treat as unplug).
+var errUARTBusy = errors.New("uart busy (OTA in progress)")
+
 // managedSerial keeps the UART open and reconnects after I/O failures or unplug.
 type managedSerial struct {
 	portName string
@@ -69,7 +73,22 @@ func (m *managedSerial) invalidateLocked(reason error) {
 }
 
 func (m *managedSerial) withPort(fn func(*serialPort) error) error {
-	m.mu.Lock()
+	return m.withPortLocked(false, fn)
+}
+
+// withPortPoll is like withPort but returns errUARTBusy instead of blocking during OTA.
+func (m *managedSerial) withPortPoll(fn func(*serialPort) error) error {
+	return m.withPortLocked(true, fn)
+}
+
+func (m *managedSerial) withPortLocked(try bool, fn func(*serialPort) error) error {
+	if try {
+		if !m.mu.TryLock() {
+			return errUARTBusy
+		}
+	} else {
+		m.mu.Lock()
+	}
 	defer m.mu.Unlock()
 
 	if m.sp == nil {
@@ -86,8 +105,19 @@ func (m *managedSerial) withPort(fn func(*serialPort) error) error {
 }
 
 func (m *managedSerial) exchangePayload(payload []byte, cmdName string) ([]byte, error) {
+	return m.exchangePayloadVia(m.withPort, payload, cmdName)
+}
+
+func (m *managedSerial) exchangePayloadPoll(payload []byte, cmdName string) ([]byte, error) {
+	return m.exchangePayloadVia(m.withPortPoll, payload, cmdName)
+}
+
+func (m *managedSerial) exchangePayloadVia(
+	portFn func(func(*serialPort) error) error,
+	payload []byte, cmdName string,
+) ([]byte, error) {
 	var resp []byte
-	err := m.withPort(func(sp *serialPort) error {
+	err := portFn(func(sp *serialPort) error {
 		var e error
 		resp, e = sp.exchangePayloadLocked(payload, cmdName)
 		return e
@@ -96,8 +126,19 @@ func (m *managedSerial) exchangePayload(payload []byte, cmdName string) ([]byte,
 }
 
 func (m *managedSerial) exchange(cmdID byte, cmdName string) ([]byte, error) {
+	return m.exchangeVia(m.withPort, cmdID, cmdName)
+}
+
+func (m *managedSerial) exchangePoll(cmdID byte, cmdName string) ([]byte, error) {
+	return m.exchangeVia(m.withPortPoll, cmdID, cmdName)
+}
+
+func (m *managedSerial) exchangeVia(
+	portFn func(func(*serialPort) error) error,
+	cmdID byte, cmdName string,
+) ([]byte, error) {
 	var resp []byte
-	err := m.withPort(func(sp *serialPort) error {
+	err := portFn(func(sp *serialPort) error {
 		var e error
 		resp, e = sp.exchangeLocked(cmdID, cmdName)
 		return e