Add maintenance mode and board status indicators

Provide BOOT-triggered USB CDC maintenance mode for file updates, plus
onboard LED activity flashes and enhanced display status information.

led.py

@@ -0,0 +1,59 @@
+"""Short APA102 activity flashes for the T-Dongle-S3's onboard RGB LED."""
+
+import time
+from machine import Pin
+
+# Verified against LILYGO's T-Dongle-S3 pin map. The APA102 uses BGR order.
+_DATA_PIN = 40
+_CLOCK_PIN = 39
+_PULSE_MS = 90
+_BRIGHTNESS = 1 # APA102 global brightness: 1 (dim) through 31 (full).
+
+class ActivityLed:
+    """Drive the single onboard APA102 LED without delaying the main loop."""
+
+    def __init__(self):
+        self._available = True
+        self._off_at = None
+        try:
+            self._data = Pin(_DATA_PIN, Pin.OUT, value=0)
+            self._clock = Pin(_CLOCK_PIN, Pin.OUT, value=0)
+            self._write(0, 0, 0)
+        except (OSError, ValueError):
+            # Keep display-less or alternate ESP32-S3 development boards usable.
+            self._available = False
+
+    def pulse(self, red, green, blue, now):
+        """Show one colour briefly; a later event simply extends the flash."""
+        if not self._available:
+            return
+        self._write(red, green, blue)
+        self._off_at = time.ticks_add(now, _PULSE_MS)
+
+    def tick(self, now):
+        """Turn the LED off after its scheduled pulse without sleeping."""
+        if (self._available and self._off_at is not None and
+                time.ticks_diff(now, self._off_at) >= 0):
+            self._write(0, 0, 0)
+            self._off_at = None
+
+    def _write(self, red, green, blue):
+        # APA102: start frame, global-brightness pixel frame, end frame.
+        self._send_byte(0)
+        self._send_byte(0)
+        self._send_byte(0)
+        self._send_byte(0)
+        self._send_byte(0xE0 | _BRIGHTNESS)
+        self._send_byte(blue)
+        self._send_byte(green)
+        self._send_byte(red)
+        self._send_byte(0xFF)
+        self._send_byte(0xFF)
+        self._send_byte(0xFF)
+        self._send_byte(0xFF)
+
+    def _send_byte(self, value):
+        for bit in range(7, -1, -1):
+            self._data.value((value >> bit) & 1)
+            self._clock.on()
+            self._clock.off()