feat(frontend): 💄 modernize ui

[amd989]

Summary

• Complete frontend migration from MUI v4 / Webpack / Yarn to React 18 + Vite + Tailwind CSS + shadcn/ui + npm
• Wired new UI to existing Flask backend — dashboard polls /api/print_status, file manager uses /api/list_files, file details/preview/upload/delete all functional with CSRF token handling
• Added test suite with vitest + @testing-library/react (36 tests covering API client, StatusIndicator, PrintProgress, PrintControls)
• Updated CI/CD — both ci.yaml and package.yaml workflows migrated from yarn to npm; added @vitest/coverage-v8 for coverage reporting
• Updated Dockerfile to build frontend with npm before packaging
• Updated docs (contributing.rst, CLAUDE.md) to reflect new stack and commands

Changes

• Replaced Material-UI, Axios, Webpack, Yarn, Storybook with Vite, Tailwind, shadcn/ui, React Query, npm
• New frontend/src/lib/api.ts — typed API client with apiFetch<T>(), CSRF support, utility functions
• Rewrote Index.tsx (dashboard) and Files.tsx (file manager) to use real API via useQuery with polling
• Rewrote FileDetailDialog.tsx with live file details, layer preview, and print/delete actions
• Added Vite dev proxy (port 3000 → 5050) for local development
• Removed all yarn artifacts (.yarn/, .yarnrc.yml, yarn.lock)
• Removed unused dependencies (~30 packages) and dead code (.babelrc, .eslintrc.js, .storybook/, mock data)
• Updated docs screenshot with new UI

Test plan

• npm run lint — passes (0 errors)
• npx prettier --check src — passes
• npx tsc — passes
• npm run build — produces dist output
• npx vitest run --coverage — 36/36 tests pass
• Manual: run poetry run mariner and verify dashboard, file list, file details, upload, print controls