fix: add a visible theme toggle to the viewer

[paolino]

Closes #80

What changed

This adds a visible dark/light theme toggle to the shared viewer controls so embedded deployments such as Cardano Knowledge Maps expose an in-page way to switch themes.

The change keeps the project aligned with the constitution:

• theme selection and persistence now live in PureScript viewer state
• JavaScript remains thin browser FFI for reading system theme and applying the resolved theme to document.documentElement
• the static HTML bootstrap only handles first-paint theme resolution from ?theme= or persisted preference so the page does not flash in the wrong theme before Halogen mounts

Implementation notes

• added theme to viewer state plus a ToggleTheme action
• render a compact theme button in the controls row
• initialize theme from URL override, then persisted preference, then system theme
• persist manual theme changes in localStorage
• keep the URL param in sync when the user toggles so links remain shareable and explicit overrides still work
• added thin FFI.Theme and extended FFI.Url with getThemeParam / setThemeParam

Verification

• nix develop --quiet -c just test
• nix develop --quiet -c just bundle-lib
• headless Chromium check against the built viewer loaded with Cardano Knowledge Maps data:
  • default load renders .theme-toggle-btn
  • clicking the button flips data-theme and updates ?theme=
  • loading ?theme=dark shows ☀ with title Switch to light theme

Reviewer focus

• confirm the control placement in the top-left viewer controls is acceptable for both app and lib variants
• confirm persisting explicit dark/light selection is preferable to cycling through an auto mode in the UI
• confirm downstream consumers only need a rebuild/redeploy to pick up the new control