Fix print2 pageSize: honour template CSS @page size when no explicit pageSize is passed

[Copilot]

Electron's webContents.print() defaults preferCSSPageSize to false, silently ignoring the @page { size } rule the vue-plugin-hiprint template engine injects into every rendered HTML. This caused the printer to fall back to its hardware default paper regardless of the template's designed paper size.

Changes

• src/print.js — added preferCSSPageSize to PRINT_WINDOW.webContents.print() (the print2 / "news" path)
• src/render.js — same fix for RENDER_WINDOW.webContents.print() (render-print path)
• README.md — documented the resulting default behaviour

Behaviour

Call	preferCSSPageSize	Paper source
print2(data, { printer }) — no pageSize	true	Template CSS @page { size } (receipt/label printers now work)
print2(data, { printer, pageSize: 'A4' })	false	Explicit JS option wins
print2(data, { preferCSSPageSize: true/false, … })	user value	User override

// Before: printer always used hardware default — template paper size ignored
WINDOW.webContents.print({ pageSize: data.pageSize });

// After: CSS @page { size } respected when no explicit pageSize is supplied
WINDOW.webContents.print({
  pageSize: data.pageSize,
  preferCSSPageSize: data.preferCSSPageSize ?? (data.pageSize == null),
});