Align identity platform UI with Kaukus style guide

Rework the identity platform’s shared frontend design to follow the Kaukus Slate + Fjord style guide while preserving server-rendered behavior, localization, authorization, and no-JavaScript usability. Acceptance criteria: - The canonical Kaukus OKLCH palette and semantic tokens drive all shared and feature styles in light and dark themes. - The application shell, typography, spacing, cards, buttons, forms, notices, badges, tables, detail views, authentication pages, and responsive behavior match the guide’s component contracts. - A clear, accessible Light / Dark / System theme control persists via the `kaukus-theme` key and works across navigation and reloads without making essential flows depend on JavaScript. - Existing localized admin, organization, user, authentication, and component-preview flows retain their semantics and functionality. - Frontend documentation and browser coverage reflect the new theme behavior and design foundation. - Project formatting, linting, unit/integration tests, asset checks, and browser tests pass.