feat(theme): Light/Dark/System, reduced motion, and one radius scale

The theme foundation DESIGN.md requires (ADR 0060):

- A design token may carry a `dark` value and a `reduced` value. The
  token stylesheet selects them in CSS alone: dark under
  prefers-color-scheme unless the host chose light, or when it chose
  dark; motion durations zero under prefers-reduced-motion unless it
  allowed motion, or when it chose reduce. `color-scheme` travels with
  the tokens. System is live, an explicit choice wins, and every module
  root follows through custom-property inheritance, with no module
  contract change.
- @ekklesia/design-tokens adds a small appearance API: system/light/dark
  and system/reduce/allow, parsers for untrusted stored values,
  resolvers, and a controller that applies the attributes immediately.
  The shell applies it before React renders. That is the hook the
  account preferences work will feed; nothing is persisted. The
  Dashboard page declares `color-scheme: light dark` so a dark system
  never flashes white. Displays stay light.
- A designed dark theme: teal-tinted charcoal, never pure black or
  white, the same teal accent.
- WCAG AA contrast is tested for 41 element pairings in both schemes.
  Two light pairings failed and are fixed visibly: text.muted darkens,
  and form controls get a new border.control (3:1).
  background.placeholder keeps skeletons visible in dark.
- Motion tokens (duration-short, duration-ambient) replace literal
  durations and every per-element reduced-motion rule. radius.scale
  multiplies sm/md/lg, unchanged at its default of 1.
- The sign-in card and module loading/error text move from literal
  colours to tokens. Guards keep raw colours and literal radii out of
  Dashboard product source and @ekklesia/ui.
- Fixes ConfirmationDialog's phrase input, which used a class that
  does not exist (`ek-input`) and so was unstyled.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture/platform-architecture.md

@@ -596,7 +596,12 @@ elements a real module dashboard cannot be built without, every value a token re
 shared interactions — a focus-trapping `Dialog` and `ConfirmationDialog`, a per-tree toast system,
 `ErrorState` with collapsed technical detail, skeletons, and `PageHeader`'s Back — and the lint
 refuses browser-native dialogs repository-wide
-([ADR 0059](decisions/0059-shared-interaction-primitives.md)). `pnpm boundaries` opens
+([ADR 0059](decisions/0059-shared-interaction-primitives.md)). Every colour token has a designed dark
+value, and the token stylesheet selects Light, Dark, or the system's scheme — and zeroes motion
+durations when motion is reduced — in CSS alone, so the Dashboard and every module root follow the
+operating system live; the host chooses an explicit appearance through a small controller, applied
+before React renders. Contrast is tested for every pairing the elements use, in both schemes
+([ADR 0060](decisions/0060-light-dark-system-theme-and-motion.md)). `pnpm boundaries` opens
 exactly two shared roles to modules **by name**, and pins both design packages beneath everything
 else. The Dashboard and display install the defaults at their document root; custom properties
 inherit into every module artifact, so a module reads `var(--ekklesia-…)` with nothing to import at