fix(issue-details): improve mobile layout

[pawankalhansh]

Summary

• make the Issue Details tabs scrollable and more compact on narrow viewports
• reduce mobile header padding/title sizing and allow long repo/title text to wrap safely
• render the bounty metadata as a two-column mobile grid and tighten conversation spacing on phones

Testing

• Verified the remote compare diff against test
• Not run locally: this environment can write files but cannot delete/rename them, so Git cannot create a working checkout (.git/config.lock cannot be renamed)

Closes #939

[pawankalhansh]

Hi maintainers, this PR fixes #939 with scoped mobile layout improvements for IssueDetailsPage. Could you please approve the workflows when you have a chance?

[anderdc]

needs screenshots