feat(issueList): Add clickable column headers to change sort order

[cleptric]

Summary

• Add ability to click on issue stream column headers (Last Seen, Age, Trend, Events, Users) to update the sort order
• Clicking a header updates the sort selector dropdown automatically (both use the same onSortChange callback)
• Active sort column is visually highlighted with primary content color
• Clickable headers show pointer cursor and hover state

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀