Right-click a selection -> "Comment on Selection..." -> opens the
comments sheet straight into composing, with a removable chip showing
what's being anchored to (clearing it falls back to a plain
document-level comment). Posts via comments.create with anchorText
set to the selection.
Only wired on rendered panes (main reader pane, Split View's preview
pane) - not the raw-markdown pane, since a selection there would
capture literal Markdown syntax as the anchor instead of clean prose.
NSMenuItem has no closure initializer, so this needed a small
target-is-self subclass (ClosureMenuItem) to wire a Swift closure
into onBuildContextMenu's NSMenu without a separate selector per
action.
Also moved the comments sheet's .sheet(...) modifier off the toolbar
badge button (which only exists once the doc already has comments)
onto the view root, since a document with zero comments still needs
to be able to open the sheet to create its first one. Reader now
refetches its own comment list after any create/reply via the sheet's
new onCommentsChanged callback, so a freshly anchored comment's inline
marker appears without reopening the document.