fix(ui): keep annotation comment popover open while it has unsaved content

[gwynnnplaine]

Warning

WIP — do not merge. Opening as a draft to share the approach. I haven't got sign-off on the implementation direction in #821 yet (only the go-ahead to take the issue). Happy to adjust before this leaves draft.

Closes #821

What

The annotation comment popover discarded an in-progress comment when the user clicked elsewhere or selected other text (e.g. to copy a snippet into their comment). Now the popover stays open and keeps its text while it holds unsaved content; the only exits are the explicit ones (Escape, the close button, ⌘/Ctrl+Enter to submit).

How it works

Two separate paths could drop the comment:

1. Outside click — CommentPopover's capture-phase pointerdown handler called onClose() on any click outside. Now it bails when the buffer is dirty.
2. New text selection (the dominant one) — selecting text made web-highlighter wrap it and fire CREATE, whose handler ran setCommentPopover(null) and opened a fresh popover, wiping the in-progress text.

The fix centers on a small dirty signal:

• packages/ui/utils/commentContent.ts — hasUnsavedContent(text, images), the single predicate for "is there content worth protecting."
• packages/ui/hooks/useAnnotationHighlighter.ts — while the popover is dirty, the highlighter is paused (stop()); a new selection then stays a plain, copyable browser selection and the existing popover is untouched. It resumes (run()) on submit/close. Wired via a new handleCommentDraftChange fed by the popover's onDraftChange.
• packages/ui/components/CommentPopover.tsx — outside-click now no-ops while dirty (mirrored into a ref so the listener doesn't re-subscribe per keystroke).
• packages/ui/components/html-viewer/useHtmlAnnotation.ts — same fix for the HTML/annotate surface, which uses an iframe message bridge instead of web-highlighter: the selection message early-returns while dirty.

Notes

• An empty popover still dismisses on outside click and still yields to a new selection — no behavior change there.
• Expanded dialog mode was already non-dismissing on backdrop clicks; unchanged.
• The shadow concern: only the text-selection comment popover is covered (the issue's surface). The plan-diff block-comment popover uses a different hover model and isn't affected.

Testing

• bun run typecheck clean; bun test 1369 pass / 0 fail (added commentContent.test.ts).
• No DOM/component test harness exists in the repo, so the click-outside/selection wiring is verified manually; only the pure hasUnsavedContent predicate is unit-tested. Flagging this as the weak spot — open to adding happy-dom if you'd want real coverage.
• Manual (plan review + HTML annotate): type a comment, select other text → popover stays, text intact, new selection is live for ⌘C; empty popover still replaced by a new selection; Escape/X/⌘↵ unchanged.