Add skeleton loader component

[220nightmore-spec]

Summary

• Adds a reusable SkeletonLoader component with text, card, table, and dashboard variants.
• Includes accessible loading status semantics (role="status", aria-busy, and configurable labels).
• Adds responsive styling, reduced-motion handling, Storybook examples, and focused unit coverage.

Closes #974

Validation

• node node_modules/vitest/vitest.mjs run src/components/SkeletonLoader.test.jsx --config vitest.no-setup.config.mjs
• node node_modules/eslint/bin/eslint.js src/components/SkeletonLoader.jsx src/components/SkeletonLoader.test.jsx src/stories/SkeletonLoader.stories.jsx
• git diff --check

Notes

• The temporary Vitest config was used only because the repository's default src/setupTests.js imports @testing-library/jest-dom, which is not declared in frontend/package.json. It was removed before committing.
• Full frontend build and Storybook build are still blocked by the repository's existing PWA/workbox configuration issues noted in related frontend validation.