Use UTF-8 byte length instead of character count or display width for message
truncation. This fixes an issue where emoji characters like 😀 (4 UTF-8 bytes,
1 character, 2 display units) caused messages to exceed the RF packet size.
Changes:
- Replace _count_display_width with byte-based length calculation
- Add _count_byte_length and _truncate_to_byte_length helper methods
- Update get_max_message_length to use 127 bytes (channel limit) not 150
- Add type hints to new methods
e2725686ef