fix(html): print one page box to a sheet

[andiwand]

A user reported that a pdf printed from the mobile apps has the wrong layout. Both apps print every document through the web view, a pdf too: the Android PdfDocumentAdapter overload has no caller. So what prints is our HTML, and the fixed-page views had no print rules.

Before

Chrome --print-to-pdf of the current output:

• The grey backdrop (#525659), the 16px gaps and the shadows print.
• The page boxes break across sheets: the 9-page Casio_WVA-M650-7AJF.pdf printed on 10 sheets, each with parts of two pages. Slides do the same: 6 slides of style-various-1.odp on 4 sheets.
• A landscape page shrinks onto a portrait sheet.
• A text page with margins gets the margins twice: the browser's and the page box's.

Change

• document.css: @media print takes the backdrop, the gaps, the padding and the shadows off .odr-pages and .odr-page-outer, and breaks after each page box.
• document.cpp: a paged view writes an @page rule with the size of the first page. A text page puts its margins on the sheet and drops them from .odr-page-inner, so text that runs past a sheet keeps them.
• pdf_file.cpp: the same print rules for .d/.p. Each page box gets a named @page for its size, so a pdf that mixes portrait and landscape pages prints each sheet in its own orientation. .p gets content-visibility:visible in print, because WebKit prints an off-screen content-visibility:auto page blank.
• frontend.cpp: the dark style is gated on screen. Before, a dark render printed a black backdrop, a shadow and a dark page, or light-grey text on white paper. Now a print always uses the light style.

Verification

I printed the output of 13 corpus files (pdf, odp, pptx, key, odg, odt, docx, md, xlsx). I used Chrome through CDP Page.printToPDF in three ways: with the CSS page size, and with the paper forced to A4 and to Letter, the way the Android print framework picks it. I also used WebKit through WKWebView.printOperation on macOS with A4 and Letter, as the nearest stand-in for the iOS print path.

• Chrome, all three ways: one sheet for each page box, in the correct orientation, with no grey. When the paper is not the page size, Chrome scales the page onto the sheet.
• WebKit: one sheet for each page box, with no blank pages. WebKit ignores @page size and only shrinks to fit the paper width. So an A4 page on Letter paper still breaks over two sheets in WebKit, as before. CSS cannot fix this: in WebKit print layout vh is the height of the window, not of the paper. On iOS, the app can choose the paper with UIPrintInteractionControllerDelegate.printInteractionController(_:choosePaper:).
• Screen: the screenshots of six samples before and after are byte-identical.
• Text without page margins and spreadsheets print as before.

I did not test on a phone.

Reference output

Every paged and every pdf output changes: a new <style> in the head, and a q class on each pdf page. So compare-html needs a reference-output update. That update is not in this PR yet.