chore: add Storybook 10 with browser-mode tests via Vitest

[zhawtof]

Summary

• Adds Storybook 10 (@storybook/react-vite) for visual development, with addons docs, a11y, and vitest.
• Wires Storybook's play() interaction tests into pnpm test via @storybook/addon-vitest's storybookTest Vitest plugin. The existing jsdom unit project is preserved; stories run headlessly in Playwright Chromium as a second project.
• Adds initial stories for BlockKitBuilder, Toolbar, Palette, and Button covering rendering smoke + interaction assertions.

Details

Config

• .storybook/main.ts — react-vite framework, addons, Tailwind v4 plugin via viteFinal.
• .storybook/preview.{ts,css} — globals + theme toolbar, plus shadcn token defaults (light/dark) so components render with real colors in the catalog.
• vitest.config.ts split into two projects:
  • unit (jsdom) — existing test/**/*.test.ts
  • storybook (browser) — Playwright Chromium via @vitest/browser-playwright, picks up src/**/*.stories.tsx

Stories

• BlockKitBuilder — Empty, WithStarterBlocks, MessageSurfaceLocked, DarkPreviewDefault, plus play() tests for Clear and AddSection-from-palette flows.
• Toolbar — 6 variants with click assertions for Send, Issues, etc.
• Palette — wrapped in DndContext, with an add-via-chevron interaction test.
• Button — 8 variant + interaction stories including disabled-state assertion.

CI

• ci.yml and publish.yml install Playwright Chromium before pnpm test. Test job timeout raised to 10 min.
• storybook-static/ added to .gitignore.

Test plan

• pnpm test — 34 passed (12 unit + 22 Storybook stories in Chromium).
• pnpm run typecheck — clean.
• pnpm run lint — clean.
• pnpm run build — package bundle unchanged (stories not pulled in).
• pnpm run build-storybook — produces storybook-static/.
• Reviewer: try pnpm storybook locally to browse the catalog.

Notes for reviewers

• Stories live alongside components (*.stories.tsx) rather than in a separate top-level directory; tsup only bundles what's reachable from src/index.ts, so they don't ship.
• @storybook/addon-vitest 10.3+ auto-provisions preview annotations, so no vitest.setup.ts is needed.
• The Storybook browser project requires Playwright Chromium; CI installs it just-in-time before pnpm test.

🤖 Generated with Claude Code