mnemonic: fix %zu format specifier in restore-flow titles

[oroderico]

Summary

• The mnemonic restore flow (word selection, confirmation, insertion, and the "restore by word number" method from Add BIP39 word-number mnemonic entry #312) used %zu for size_t arguments in on-screen title snprintf() calls.
• On at least ESP32-S3 boards this renders the raw zu characters instead of the value (e.g. Word zu/zu), observed on a Waveshare S3 Touch LCD2 while testing the word-number restore flow.
• Cast the arguments to unsigned int and use %u instead, which is unambiguous on this 32-bit target regardless of libc/format-string quirks.

Test plan

• Built and flashed on a Waveshare S3 Touch LCD2, confirmed titles now render correctly (Word 1/24, Select word N, Insert word N, Confirm word N) in both the normal typed restore and the word-number restore flow.
• Would appreciate confirmation this was never visible on other (ESP32 classic) boards, to help narrow down the root cause.