Follow users: pill in detail view + glass pill on profile

Created by: jedmund

Summary

• Adds /v1/user/follow/add and /v1/user/follow/delete to PixivEndpoint and a shared FollowState observable + two button variants (FollowPillButton, FloatingFollowPill).
• Detail view: Follow pill sits at the trailing end of the metadata card's user row, paired with a compact 38pt share icon (both on secondarySystemBackground, explicit Color(.label) glyph). The old full-width bottom share button is gone — share lives inline with the user it belongs to.
• Profile view (pushed): glass Follow pill in the floating chrome's trailing slot, so it scrolls with the user sibling-to the back chevron. Hidden while userDetail hasn't resolved, and fully suppressed for isCurrentUser.

IllustDetailViewController fetches .userDetail(userID:) per current item to resolve is_followed, cancels the in-flight fetch on swipe-page, and does optimistic toggle + rollback on failure. ProfileView seeds the state from the detail it already fetches and resets to .unknown on pull-to-refresh.

Test plan

• Open a detail from Home → tap Follow → pill flips to "Following" with a plus→checkmark morph
• Tap Following → pill flips back to Follow; confirm the real Pixiv state persists across app relaunch
• Swipe-page in detail while follow POST is in flight — state snaps to the new author without clobbering on late response
• Push profile (tap user row in detail) → glass Follow pill appears top-trailing once header loads, and scrolls with chrome
• Follow / unfollow on profile → persists after popping back to detail (next detail open re-fetches)
• Open your own profile via Bookmarks tab → Follow pill is not shown
• Kill network mid-POST → button rolls back; confirm no visual flash before the revert
• Dark mode: both follow pill and share icon render with white glyphs, no blue tint
• Tap share icon → standard iOS share sheet with https://www.pixiv.net/en/artworks/<id>

🤖 Generated with Claude Code