feat(viewer): dark mode + system monospace font (#304) (#312)

Enhance the embedded viewer stylesheet per #304:

- Add automatic dark mode via `prefers-color-scheme` (no JS/toggle).
  The full colour palette — surfaces, text, borders, accents, task/badge
  brand colours — lives in `:root` custom properties, with a single dark
  block overriding the values. Light-mode colours are unchanged.
- Declare `color-scheme: light dark` so native scrollbars and form
  controls follow the active theme.
- Use the system monospace stack (`ui-monospace, ...`) via a `--mono`
  variable, replacing the hardcoded font stacks.
- Dark tokens chosen to clear WCAG AA (>=4.5:1) for small text, and dim
  brand colours (task-main, task-default) lifted for the dark surface.

Verified by rendering all three viewer pages in light and dark with a
headless browser: computed colours, contrast ratios, resolved theme
variables, and `color-scheme` asserted programmatically.

Author: chethanuk