Below the 952px container-query breakpoint the right-margin comment column is
hidden, so tapping a highlighted passage previously did nothing — inline-comment
threads were unreachable on narrow windows and phones. This adds a popover that
surfaces them there.

- Extract a shared CommentPanel from CommentSidebar (a `pin` prop gates the
  margin-column padding/visibility pinning), wrapped by the wide aside and the
  new CommentPopover.
- Track a `narrow` layout signal from a ResizeObserver on the layout container
  (the pattern Breadcrumbs already uses), gating the aside vs the popover so the
  same panel mounts in exactly one place.
- CommentPopover: a free-mode Popover anchored to the highlight in article
  coordinates — fixed 288px width (matching the sidebar card), centered on the
  highlight and clamped on-screen, with a gap below it at the page bottom.
- Dismissal handled in-component so it can exempt highlight clicks (tap another
  highlight to switch, not close) and defer Escape to CommentForm (blur the
  reply/draft field, including mid-IME-composition, instead of tearing down the
  thread). Labelled role="group" "Comments" (non-modal; no over-promised dialog
  focus semantics).
- View/reply/resolve/delete, compose-new, n/p navigation, and the r focus-reply
  shortcut + per-thread reply drafts all work in the popover.

Unit tests for the breakpoint and clamp helpers; Playwright e2e covering open,
dismiss, compose, switch, n/p, r, reply-draft persistence, and the wide-screen
control. svelte-check clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>