Add setting for entering instance address | Update default instance from "lingva.ml" to "translate.plausibility.cloud"

[BlackRock-99]

The program has been non-functioning during the last months, since "lingva.ml" went offline.

This leads to 2 requests:

• To make it possible to set instance address in Settings manually, so that this situation doesn't repeat itself in the future.
• To set the program's default instance to "translate.plausibility.cloud".

Hope to see these changes in the future update

[aluyama]

Healthy instances (Today)

• https://lingva.garudalinux.org
• https://lingva.frontendfriendly.xyz
• https://lingva.retiolus.net

[BlackRock-99]

• https://lingva.garudalinux.org

this instance is cloudflared and therefore must not be used