Implement character-sending loop and backspace retry logic

[andyvn1]

Implement the core typing logic in the /key_press_result callback:

• On success: advance message_index, send next character
• On failure: send "BACKSPACE" first, then re-send the failed character after the backspace succeeds
• Guard against sending a new character while still waiting for a result (waiting_for_result flag)
• Example flow for message "MARS" with a typo on "R":

send "M" → success → send "A" → success → send "R" → fail
→ send "BACKSPACE" → success → send "R" → success → send "S" → success → done