fix(viewer): show Add-comment popover for selections released outside the article

[yumike]

Problem

Selecting a line's first word by dragging right-to-left releases the mouse to the left of <article>, so the article-scoped onmouseup never fired and the "Add comment" popover never appeared. The same happened for any selection released past the article's right edge or below it.

The direction-dependence is the tell: Range.getBoundingClientRect() is direction-independent, so a positioning bug would break both directions — only the mouse-release location differs, which is the event-delivery problem.

Fix

Move selection capture into useSelectionPopover, which already owns the document-level selectionchange dismiss listener:

• Adds a sibling document-level mouseup listener (gated on comments.enabled, capture-only, cleaned up on teardown) that captures a non-collapsed selection whenever it lies inside the article — so a release anywhere is caught.
• Removes the public capture() method (single owner now); the article handler keeps only the collapsed-click → highlight toggle.
• The Add-comment button preventDefaults its mousedown to keep the live selection/focus intact on click.

Edge cases hardened in review

• A selection extended past the article boundary (which never collapses) now drops the popover instead of stranding it at the old anchor.
• Disabling comments clears any captured selection, so it can't re-surface stale if comments are re-enabled mid-session.

Testing

• New hook unit tests (jsdom) covering capture-on-document-mouseup, in-article containment, gating/listener-detach, dismiss, null-on-outside, and clear().
• New real-mouse Playwright e2e that drags right-to-left releasing in the gutter, asserts the popover appears, then real-clicks it to open the draft. (The existing synthetic selectText helper dispatches a mouseup on <article> and can't reproduce this.)
• Full viewer unit suite (598) green; svelte-check and eslint clean; existing comment e2e unaffected.

🤖 Generated with Claude Code