## Summary
- adds Mantine core/hooks/form/modals/notifications plus the required PostCSS setup
- wraps the web app and shared test renderer in a Veritas Mantine provider with modals, notifications, and color-scheme bridging
- defines the v5 Mantine theme, status colors, density defaults, breakpoints, and layout shell primitives
- keeps the existing `.dark` class contract active while migrated and unmigrated surfaces coexist
- documents the foundation conventions and bundle impact for the v5 UI migration
Closes#415.
## Verification
- CI: Build
- CI: Lint & Type Check
- CI: Security Audit
- CI: Workspace Unit Tests
- `pnpm --filter @veritas-kanban/web test -- mantine-theme`
- `pnpm --filter @veritas-kanban/web typecheck`
- `pnpm --filter @veritas-kanban/web test`
- `pnpm lint:budget`
- `pnpm audit --prod --audit-level=high`
- `pnpm build`
- `./node_modules/.bin/prettier --check docs/UI-MANTINE-MIGRATION.md web/index.html web/postcss.config.cjs web/src/__tests__/mantine-theme.test.tsx web/src/__tests__/test-utils.tsx web/src/components/layout/mantine-shell.tsx web/src/hooks/useTheme.ts web/src/main.tsx web/src/theme/color-scheme.ts web/src/theme/mantine-theme.ts web/src/theme/MantineRoot.tsx web/vite.config.ts web/package.json`
- `git diff --check`
- Browser smoke: `http://127.0.0.1:3000/` rendered the setup page with Mantine CSS variables, dark color scheme, no new console errors, and no horizontal overflow at 1280x720 or 390x844
## Notes
- Production audit still reports the existing 3 moderate advisories; the high-severity gate passes.
- Build now emits explicit Mantine vendor assets: `vendor-mantine-CvPmQ6ZW.css` at 214.56 kB / 31.59 kB gzip and `vendor-mantine-JZsLAwaX.js` at 148.11 kB / 45.81 kB gzip.
