[applets/weather] Use unit description from KUnitConversion

Summary:
Use unit description directly from KUnitConversion instead of making a new string.
This avoids duplicating translation strings and improves consistency.
The units name is now `description` (`symbol`) also for consistency with krunner

Test Plan:
No behavior changes within the applet
`Configure` -> `Units` shows translated `description` (`symbol`) for every unit

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24953