Add Export / Share functionality to Compare page

[AditthyaSS]

The Compare page is powerful but has no way to share or save results. Add:

Copy as Markdown button — generates a formatted table for pasting into GitHub issues, READMEs, etc.
Shareable URL — encode selected repos in query params (e.g., /compare?repos=facebook/react,vuejs/vue,sveltejs/svelte) so the comparison can be shared via link
(Nice-to-have) Export as PNG — capture the comparison table as an image
Issue 12

[Payal29037]

Hi! I'd like to work on this issue as part of ECSoC. Could you please assign it to me?

[Flamingo27]

I would like to work on this issue as part of ECSoc 2026, kindly assign me.