#### Brief overview of PR changes/additions
Restores support for several East Asian text encodings (Big5,
Big5-HKSCS, GBK, GB18030 and EUC-KR) that stopped working in 4.21.0 on
some builds. Affected players saw garbled characters on screen and got a
"no codec found" error when trying to send text.
#### Motivation for adding to Mudlet
A change in 4.21.0 left these encodings unavailable on builds whose Qt
was not shipped with an extra text-conversion library, which broke
connections to many Chinese and Korean games (for example on macOS).
This change makes Mudlet fall back to a conversion that is always
available, so the encodings work everywhere again.
#### Other info (issues closed, discussion etc)
Closes#9344. Also adds automated tests so these encodings keep working
going forward.
<img width="1407" height="497" alt="Screenshot 2026-06-19 at 4 22 30 PM"
src="https://github.com/user-attachments/assets/8b4d172f-867d-4aa8-9e75-a0c0a4e56b3f"
/>
---------
Signed-off-by: Mike Conley (aka Tamarindo) <sousesider@gmail.com>
