Fix bot mode hang on WiFi connection (closes #5)

[semichcsc-byte]

Problem

When the user selects the Chess Bot (Human vs AI) mode, the board hangs after Connecting to WiFi... with no further serial output. After ~50s the failure animation runs (board flashes red 5x) as if the WiFi credentials were wrong, but they are correct — same SSID/password works for any other client.

This matches the symptoms reported in #5 exactly:

Making API request to Stockfish...
Failed to connect to Stockfish API

Root cause

WiFiManager::begin() runs in setup() and puts the WiFiNINA module into AP mode (OpenChessBoard / chess123) so the user can pick a game mode from a browser. When the user later picks the bot mode on the physical board, ChessBot::connectToWiFi() immediately calls WiFi.begin(SECRET_SSID, SECRET_PASS) to switch to STA mode.

WiFiNINA on the Arduino Nano RP2040 Connect (and the Nano 33 IoT / MKR WiFi 1010) cannot run AP and STA simultaneously. With the AP still listening, WiFi.begin() silently never reaches WL_CONNECTED, the 10×5s retry loop times out, and the bot mode dies.

Fix

Explicitly tear the AP down before opening the STA connection:

WiFi.disconnect();
WiFi.end();
delay(2000);

Then proceed with WiFi.begin() as before. WiFi.end() releases the radio cleanly and delay(2000) lets the WiFiNINA firmware finish reinitialising before the next mode change.

Verification

Tested on Arduino Nano RP2040 Connect with WiFiNINA firmware 3.0.1 and the official OpenChess.ino (with #define ENABLE_WIFI):

=== Starting Chess Bot Mode ===
Bot Difficulty: Medium (Depth 10)
Connecting to WiFi...
Tearing down AP mode before connecting as station...
Attempting to connect to SSID: <my-ssid>
Connection attempt 1/10 - Status: 3
Connected to WiFi!
IP address: 192.168.0.99
WiFi connected! Bot mode ready.
...
Making API request to Stockfish...
=== RAW API RESPONSE ===
{"success":true,"evaluation":0.12,"mate":null,"bestmove":"bestmove d7d5 ponder a2a4",...}
Bot move: d7d5

Bot connects on the first attempt, Stockfish API responds, gameplay proceeds normally.

Trade-offs

• +10 lines, 1 file (chess_bot.cpp)
• The OpenChessBoard AP is no longer reachable once bot mode starts. This is unavoidable on WiFiNINA hardware and matches user expectation: once a mode is picked, the AP is no longer needed.
• No behavioural change in non-bot modes.