DOCX/XLSX preview: add renderDocx (docx-preview) and renderXlsx (SheetJS)
to shared/preview-lib.js — the natural home alongside renderTiff/
renderZipListing, reusable by every tool. browse dispatches Office files
to them in both the inline pane and the pop-out window via the existing
preview.isOffice() check, and browse/build.sh now bundles the
docx-preview + xlsx vendors. Renderers degrade to a friendly message if a
tool doesn't bundle the vendor.
Overflow fix: .md-shell used `grid-template-columns: 280px 1fr`. A bare
`1fr` is `minmax(auto, 1fr)`, whose `auto` floor is the editor's
min-content width (Toast UI's toolbar) — so the content track refused to
shrink and the whole shell overflowed #previewBody as the window narrowed
instead of the editor reflowing smaller. Switch both tracks to
minmax(0, …) in the CSS and in the three JS spots that rewrite the
columns on sidebar-drag, and give .md-shell__sidebar min-width: 0.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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