Extract a container-query-based gradient mixin and apply it across card, teaser, document-list, election-form, and newsletter-teaser. Migrate deprecated theme-color-* CSS variables to the updated design tokens (feedback-error, text-accent-default, swatches-*, color-surface-inset).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

- @storybook/addon-vitest (inkl. Playwright Chromium + Coverage-V8)
- @storybook/addon-a11y für Barrierefreiheitsprüfungen
- @chromatic-com/storybook für visuelle Regressionstests
- vitest.config.ts erstellt (Storybook-Testprojekt mit Playwright-Browser)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Webpack → Vite (Startzeit: mehrere Minuten → ~300ms)
Sass Deprecation Warnings beseitigt (@import → @use as *, 93 Dateien)
require() → ESM import in 8 JS-Dateien
package.json von ~35 auf 6 devDependencies bereinigt