feat(find): open the preview find bar on the selected text

[PathGao]

Reworked from #231, which bundled this with a Copy-as-Markdown command. The copy half is not landing (reasoning in that thread); this is the half that closes a real gap. The commit keeps @junglesub as the author.

What changes

Select a word in the rendered preview, press Cmd/Ctrl+F, and the bar opens with that word already in it.

The editor pane has behaved this way for as long as it has used Monaco — seedSearchStringFromSelection defaults to seeding — so the two panes disagreed about the same keystroke, and the preview was the one that made you retype the word you were already pointing at.

Decisions

The seed is selection.toString(), not the text of one node. A selection crossing **bold**, a link or inline code spans several text nodes, and those are the words a reader is most likely to have highlighted. Reading a single node would have left the feature off in the commonest case.

Single-line selections only — Monaco's own rule in the other pane, so the app agrees with itself about one keystroke instead of growing a second rule. A paragraph in the search box matches nothing and buries the query the user was about to type.

Nothing to seed leaves the previous query alone rather than clearing it: a caret with nothing selected, a selection in a modal or the tab strip, whitespace. A repeated Cmd/Ctrl+F still just re-focuses and re-selects the box (#559).

setQuery is the assignment and nothing else. The existing query effect schedules the search, so a seed goes through the same debounce and the same not-yet-open guard as typing. applyHighlights and the scroll-restore path are untouched.

Not included: starting from the selected occurrence rather than the first match. #231 spent ~110 lines on it — a pendingActiveRange / getRangeMatchIndex / getClosestMarkIndex fallback chain, an untrack change to the sanitizedHtml effect, and an extra reapply() in the render path — inside a tree walk that mutates the nodes it is walking. It can come back on its own merits.

Testing

npm test — 977 pass. npm run check — clean.

The decision of whether to seed is the whole feature, so it lives in src/lib/utils/findSeed.ts as a pure function the test imports and runs, plus one source-shape line for the wiring (a pure function nothing calls is dead code). Checked by mutation: dropping the single-line rule fails it, and so does dropping the containment guard.

Manually verified on macOS with a debug build: selecting across bold and inline code, selecting in the tab strip, repeated Cmd+F with no selection, and split view.

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