Add configurable SOS message support for T-Echo

[pedrocaldeira66]

This PR adds a configurable SOS/Help message feature for T-Echo devices.

The feature allows the device to send a predefined emergency/help message from the device UI/button flow.

Current behaviour:

• Uses a configurable SOS message with {gps} placeholder support.
• Replaces {gps} with the current GPS coordinates when available.
• Appends a Google Maps link when GPS coordinates are available.
• Falls back to GPS sem fix when no GPS fix is available.
• Sends to the configured SOS channel slot.
• Falls back to Public / slot 0 if the configured channel is not available.
• Includes optional debug logging controlled by SOS_DEBUG.

This is intended as a lightweight field/off-grid helper for outdoor MeshCore use.

Future possible improvements:

• Optional acknowledgment/reply handling.
• Configurable retry interval.
• ATAK / TAK emergency marker integration.

[pedrocaldeira66]

Additional testing update:

Tested successfully on:

• LilyGo T-Echo
• Heltec Wireless Tracker
• Heltec V3

On all three devices:

• SOS page appears correctly in the UI.
• Long press from the SOS page sends the message.
• Triple-press shortcut also sends the SOS message.
• Messages are received across bridged 868 MHz / 433 MHz MeshCore networks.