theme(fix): add transform.Plainify to the summary body content of the share links (#758)

You definitely want to **add Plainify transformer first**, because the
body content will become a HTML encoding mess. I left `HTMLEscape` there
as well, just be sure for security reasons. But if you want, I can
remove the second transformer.

Like a tweet on X will look like this:


![image](https://github.com/user-attachments/assets/73ab37b0-decc-47a4-8bbd-cf43cbef1641)


After implementing `transform.Plainify`, it will look much better:


![image](https://github.com/user-attachments/assets/962ba515-72ae-4b46-827f-2c477358d065)

(ignore the red, I don't have X premium pro blabla, it's red because I
hit the max limit of chars)

Signed-off-by: Melroy van den Berg <melroy@melroy.org>