fix: print the canvas exactly, with no added whitespace

[vibhavkatre]

Three things in the print stylesheet put white on the page or changed how the
diagram looked.

/* before */
@page{size:1280px 720px;margin:0}
html,body{margin:0;padding:0}
svg{display:block;width:100%;height:auto}

height:auto sized the drawing from its own aspect ratio rather than the page.
Whenever the page box was not exactly the canvas ratio — which is whenever the print
dialog falls back to the user's paper size — the leftover page height printed as a
white band. That is the reported extra whitespace.

html and body had no height, so a 100% had nothing to resolve against.

Background colours are dropped when printing unless asked not to, so a coloured
canvas came out white. That is the other half of "doesn't match the canvas exactly".

/* after */
@page{size:1280px 720px;margin:0}
html,body{margin:0;padding:0;width:100%;height:100%}
svg{display:block;width:100%;height:100%}
*{-webkit-print-color-adjust:exact;print-color-adjust:exact}

The same builder serves Print and the PDF fallback, so both improve.

Verification

I captured the exact document the app hands the print window, then rendered it the
way a printer does — Chromium honours @page with preferCSSPageSize:

• the PDF page comes out 960 x 540 pt, which is exactly the 1280 x 720 px canvas,
  not the default paper
• the drawing runs edge to edge: 0,0 to 1280,720 against a 1280x720 page
• the amber canvas background survives the print instead of coming out white

Rendered page, for comparison with the canvas: the background fills the sheet and
both corner shapes sit where they do on screen.

• yarn test — 916 pass, 6 new checks in printDocument.test.js
• yarn build — clean

Closes #227