web: support both light and dark color schemes

[acinader]

Depends on the css scrub (#2705).

hledger-web's UI is light-only. Users who keep their OS in dark mode
(or have it switch automatically at night) get a bright page, and
browser-drawn widgets (suggestion popups, date picker, scrollbars) can
come out mismatched with the page around them.

A first small step goes into #2702: declaring color-scheme: light (https://web.dev/articles/color-scheme) tells the browser the
page is light-only, so it draws its widgets light to match. That fixes
the mismatched-popup problem but leaves dark-mode users with a bright
page.

This issue proposes going further and actually supporting both
schemes. Roughly:

• declare color-scheme: light dark
• define the palette in one place (css custom properties or the newer
  light-dark() function,
  https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/light-dark)
• pick dark equivalents for the UI colors, including the
  positive/negative amount colors, sidebar, and register/journal rows
• respect prefers-color-scheme with no toggle to start; a manual
  override could come later if wanted

No javascript is needed for any of this. The main work is choosing a
dark palette that keeps the reports readable and the red/green amount
colors distinguishable.