Replace unsafe (wchar_t*)pchText casts with uint16_to_wstring() in all
keyboard callbacks. The direct cast is incorrect on platforms where
wchar_t is 4 bytes (Linux/macOS). New helpers in StringHelpers route
through u16string for proper UTF-16 to wchar_t conversion.
- Add uint16_len, uint16_to_u16string, uint16_to_wstring to StringHelpers
- Fix casts in AnvilMenu, CreateWorldMenu, DebugCreateSchematic,
DebugSetCamera, LaunchMoreOptionsMenu, SignEntryMenu
- Truncate sign text to 15 chars in SignEntryMenu
- Move m_bIgnoreInput reset after if-block in LaunchMoreOptionsMenu
