Add compatibility with SAMD21 properly

[zshzebra]

#ifdef ARDUINO_ARCH_SAMD
#define MT_WIFI_SUPPORTED
#endif

Currently, WiFi support is enabled solely based on ARDUINO_ARCH_SAMD macro,
which is too broad and causes linking errors on non-WiFi SAMD boards.

Instead of automatic detection, this change requires users to explicitly
enable WiFi support through a configuration flag. This prevents unexpected
linking errors on SAMD boards that don't have WiFi capabilities (like
Arduino Zero or other M0-based boards without WiFi hardware).

The change affects:

• Removes automatic WiFi detection based on architecture
• Users must now define MT_WIFI_SUPPORTED explicitly
• Prevents linking errors on non-Feather M0 SAMD boards

Other Changes

• Setup baud rate

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