Fix Unicode values in external settings

[Jacksper13]

## Summary

• encode serialized external settings as UTF-8 before calculating capacity and padding
• reject oversized settings before selecting or modifying a flash slot
• distinguish genuine settings-capacity failures from unexpected save failures
• add save/load coverage for Korean and smart-quote multisig names and slot-size boundaries

Root cause

`ExtSettings.save_impl()` calculated padding with the character length of the JSON string, while AES encryption wrote its UTF-8 bytes. Multibyte names therefore caused more bytes to be written than the fixed slot allowed, and the final checksum write asserted.

The encoded payload is unchanged, so existing settings remain compatible and no storage migration is required.

Fixes #658