The rendered Markdown preview had no way to search: the Electron desktop
shell implements no find-in-page at all, and CodeMirror's search panel only
exists in edit mode, so Ctrl/Cmd+F in the preview was a dead shortcut (web
browsers happen to find plain-DOM text natively, but desktop does not).
Adds a compact find bar for the rendered preview (Ctrl/Cmd+F or the search
button): case-insensitive match highlighting with a live count, Enter /
Shift+Enter and arrow buttons to navigate matches, Esc to close. Matches
are wrapped in <mark> elements and re-applied via MutationObserver when the
markdown renderer re-morphs the container (theme/content changes); svg
(mermaid) and script/style text is skipped. The pure match-range logic is
unit-tested.
Fixes#2401
Serhii Dziupin
17d5b90d83