conversation-activity-filter - Fix height and placement - #10372

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

Follow-up to #10325, where the widget ended up on the left of the PR sticky header.

• PR sticky header: order: 10 on the widget in the title area, so it comes after the title
• PR sticky header: GitHub's .prTitleArea has min-width: 100vw (1280px from 1012px up), which is wider than the window below 1280px and pushed the right-aligned widget off screen. It is now capped at min(100vw, 1280px) while the widget is present
• All headers: the button no longer uses Button--small, so its height matches "Copy link" everywhere
• To keep it compact without Button--small, one self-contained rule sets Primer's small inline padding and gap, and turns .Button-content into a flex row so empty grid areas can't add space. The empty label is also no longer rendered

Test URLs

#4030 (scroll down to show the sticky header)
#10169 (scroll down to show the sticky header)

Checked

• Type-checks, lints and passes the format check
• Button rules read from the published @primer/view-components CSS (0.53.5), not GitHub's live stylesheet, which I can't reach

Not checked

• Logged in, so none of the visual result: the widget's position in the PR sticky header, the height against "Copy link", and the padding and gap. the padding and gap values have fallbacks (8px and 4px) in case the Primer variables aren't defined
• The events label size: it is now the medium font size, since Button--small also set the font size
• Screenshots: none yet

Reviewer tasks

• PR sticky header: widget is on the right, after the title, and visible at window widths below 1280px
• Issue and PR headers, sticky and not: widget has the same height as its sibling buttons
• Icon-to-triangle gap and inline padding look as before, in all three filter states (including the events label)