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.

boot.py

@@ -1,6 +1,13 @@
-"""Configure native USB HID before MicroPython starts main.py."""
+"""Select USB HID or local USB-REPL maintenance mode before main.py."""
 
-from hid import initialise
+import maintenance
 
-# Keep the configured USB device and its callbacks alive for the whole session.
-hid = initialise()
+# Press BOOT during this short window after a normal reset to retain the stock
+# USB CDC REPL. Holding BOOT while resetting still enters ESP download mode.
+maintenance.check_button()
+
+if not maintenance.active:
+    from hid import initialise
+
+    # Keep the configured USB device and its callbacks alive for the session.
+    hid = initialise()