* Fix PDF export sizing chart to zoomed viewport
downloadPdf() read the PDF page size from getSvgDimensions(), which returns
the live #chartSvg width/height attributes. Those carry the on-screen zoomed
size (chart_wrapper.ts writes size * scale on every d3-zoom event), not the
true chart size. The SVG drawn into the page instead carried the true
dimensions (getStrippedSvg divides by the zoom scale), so the PDF page and
its contents mismatched: zooming in produced a giant page with the tree in
one corner, zooming out produced a truncated strip. See issues #260 and #18.
Fix: have getSvgContentsWithInlinedImages() return the chart's true width
and height (read from the single stripped SVG clone it already builds), and
have downloadPdf() consume those for both the jsPDF page format and the
draw target. downloadSvg()/downloadPng() were already correct and only need
to destructure contents from the new return shape. The buggy helper
getSvgDimensions() is removed.
* Add regression test for PDF export page size
Zooms the chart, exports a PDF and asserts the PDF MediaBox matches the
chart's true (unscaled) dimensions and not the live zoomed ones. Verified
to fail on the pre-fix code (pdfW == liveW) and pass after the fix
(pdfW == trueW).
* Apply prettier formatting to chart_export.spec.ts
