Threema single-chat ZIP exports

[JeromeMaag]

Idea

Add support for Threema single-chat ZIP exports.

Why

Threema can export individual chats as ZIP files like WhatsApp. Supporting this format would make the tool useful even when no full Threema database export is available.

Proposed behavior

Select a Threema single-chat ZIP as input and export it through the existing output flows.

Notes

Added as separate Threema import path and should not replace the existing Threema database import.