feat: Add v0.2 remote management — UDP commands, LED, NVS persistence

Firmware (app_main.c):
- UDP command listener on port 5501 (configurable via Kconfig)
- Commands: REBOOT, IDENTIFY, STATUS, RATE, POWER
- LED status indicator on GPIO2 (off/slow/fast blink/solid)
- NVS persistence for send_rate and tx_power settings
- Refactored ping to support stop/restart for dynamic rate changes
- TX power control via esp_wifi_set_max_tx_power()

Pi-side tooling:
- tools/esp-cmd: standalone Python3 UDP client for device management

Tested on amber-maple (ESP32 v3.1) with ESP-IDF v5.5.2.

tools/esp-cmd

@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+"""Send management commands to ESP32 CSI devices over UDP."""
+
+import socket
+import sys
+
+DEFAULT_PORT = 5501
+TIMEOUT = 2.0
+
+USAGE = """\
+Usage: esp-cmd <ip> <command> [args...]
+
+Commands:
+  STATUS              Query device state (uptime, heap, RSSI, tx_power, rate)
+  REBOOT              Restart the ESP32
+  IDENTIFY            Blink LED solid for 5 seconds
+  RATE <10-100>       Set ping frequency in Hz (saved to NVS)
+  POWER <2-20>        Set TX power in dBm (saved to NVS)
+
+Examples:
+  esp-cmd 192.168.1.50 STATUS
+  esp-cmd 192.168.1.50 RATE 50
+  esp-cmd 192.168.1.50 POWER 10
+  esp-cmd 192.168.1.50 REBOOT
+  esp-cmd 192.168.1.50 IDENTIFY"""
+
+
+def main():
+    if len(sys.argv) < 3 or sys.argv[1] in ("-h", "--help"):
+        print(USAGE)
+        sys.exit(0 if sys.argv[1:] and sys.argv[1] in ("-h", "--help") else 2)
+
+    ip = sys.argv[1]
+    cmd = " ".join(sys.argv[2:]).strip()
+
+    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+    sock.settimeout(TIMEOUT)
+
+    try:
+        sock.sendto(cmd.encode(), (ip, DEFAULT_PORT))
+        data, _ = sock.recvfrom(512)
+        print(data.decode().strip())
+    except socket.timeout:
+        print(f"ERR: no reply from {ip}:{DEFAULT_PORT} (timeout {TIMEOUT}s)", file=sys.stderr)
+        sys.exit(1)
+    except OSError as e:
+        print(f"ERR: {e}", file=sys.stderr)
+        sys.exit(1)
+    finally:
+        sock.close()
+
+
+if __name__ == "__main__":
+    main()