[Issue] Missing bounds checking in read_from_file()

[RomanAlexandroff]

Problem

read_from_file() copies LittleFS file content into fixed-size RTC buffers using strcpy() without knowing the destination size.
Corrupt or manually edited files can overflow rtc_g.Secret, rtc_g.chat_id, or other buffers.

Expected behavior

File restore should never write past destination buffer size.

Suggested fix

Pass destination buffer size into read_from_file() and use bounded copy with explicit null termination.