conversation-activity-filter - Add to PR sticky header - #10325

Created by Claude, will be reviewed by @fregante first.

closes #10228, redone from scratch to address the review there: one selector for the sticky header, no :is(), the PH_Actions order rule and the anchor change are dropped, and the breakpoint is 1075px.

Follow-up to #10187. Related: #10169

• Observes the PR sticky header title area, so the widget is also there while scrolling
• Shows it after the title there with a single order rule
• Hides it there when the window is 774px wide or narrower
• Narrow-layout rule now applies up to 1075px

Test URLs

#4030 (scroll down to show the sticky header; narrow the window to check the layout)
#10169

Checked

• Type-checks
• Selector and class names taken from the logged-out DOM of Test-only PR with every event #4030

Not checked

• Logged in, so none of the visual result: the sticky header's actual contents, spacing next to the truncated title, and the narrow-layout order
• The 1075px value is the one from the review of conversation-activity-filter - Fix sticky header and mobile alignment #10228, and 774px was given to me; I did not measure either
• Screenshots: none yet

Reviewer tasks

• Sticky header on a PR: widget appears after the title, header not resized
• Sticky header at 774px or narrower: widget is hidden
• Window narrower than 1075px on a PR and on an issue: primary button (Merge, New issue) stays first