feat(ui): add Storybook renderer baseline

[Astro-Han]

Summary

Adds a renderer-aligned Storybook baseline for Maka UI primitives.

Why

Tracks #363. This gives frontend file-governance work an isolation gate: extracted primitives can be mounted outside the app shell while still using the renderer's Vite, Tailwind, CSS, token, and theme substrate.

Scope

Changed:

• Added @maka/desktop Storybook scripts and dev dependencies: storybook and @storybook/react-vite.
• Added .storybook/main.ts with React+Vite, Tailwind v4, and @maka/ui source aliases matching the renderer setup.
• Added .storybook/preview.tsx that imports renderer styles.css and provides light/dark plus palette root decorators.
• Seeded packages/ui with a small primitive story fixture covering Button, Alert, Empty, and Spinner.
• Added a source contract test that keeps Storybook out of mandatory build / test while pinning the baseline setup.

Not included:

• No visual redesign.
• No mandatory CI gate for Storybook yet.
• No app renderer stories yet; the first baseline only scans packages/ui stories.

Verification

• npm run -w @maka/desktop build-storybook
• npm run -w @maka/desktop typecheck
• npm run -w @maka/desktop build
• cd apps/desktop && node --test "dist/main/**/*.test.js" → 1621/1621 passing
• npm run -w @maka/desktop build:main && cd apps/desktop && node --test dist/main/__tests__/storybook-baseline-contract.test.js
• Storybook dev smoke: http://127.0.0.1:6006/index.json includes Primitives/Storybook Baseline

User-facing impact

None. This is dev tooling only; desktop app runtime and production build behavior are unchanged.

Reviewer notes

npm run -w @maka/desktop test currently stops in pretest at scripts/check-console.mjs because packages/headless/src/harbor-cli.ts has pre-existing console.error call sites. The desktop node test suite itself passes when run from apps/desktop after build:main.

npm audit reports 4 advisories against current direct/transitive tooling dependencies (vite, undici, esbuild, @babel/core). This PR does not broaden into dependency remediation.