Add optional JSON serial API for external controlAdd optional JSON serial API

[samuelcoustet]

This PR adds an optional JSON line-based serial API for ATS Mini ESP32-S3 + SI4732.

Goals:

• external control from Python / Flask / desktop applications
• preserve backward compatibility with the existing serial protocol
• minimal code changes
• no impact on UI responsiveness

Features:

• JSONL serial protocol
• automatic JSON detection using {
• protocol versioning
• read-only mode by default
• token-protected read-write mode
• automatic RW timeout
• lightweight and full status endpoints
• frequency/mode/volume/BFO control
• seek support
• busy/event reporting

Implementation details:

• adds JsonAPI.h / JsonAPI.cpp
• minimal changes to Remote.cpp
• no modification of Remote.h
• existing serial protocol remains unchanged

Safety/stability:

• blocking scan API intentionally omitted
• seek operations expose busy/event state
• API designed to avoid accidental radio control on USB connection

Tested:

• JSON parsing
• coexistence with existing serial commands
• read-only protection logic
• serial auto-detection logic

Future possible extensions:

• async events
• websocket bridge
• web dashboard integration
• non-blocking scan engine

[samuelcoustet]

Compiled successfully with Arduino IDE after installing required libraries.

Environment:

• Arduino IDE 2.x
• esp32 by Espressif Systems
• TFT_eSPI
• PU2CLR SI4735
• ArduinoJson

Current status:

• JSON API compiles correctly
• Event subscription extension compiles correctly
• No build errors detected