deploy: Vercel-ready build:site that bundles docs + playground

[joelshejar]

Summary

Step 6 + 7 of the docs-site work, in one PR because they're tightly coupled. Sets up the Vercel deploy so a single project serves both the docs and the playground from `tabmesh.dev`.

What changes

Playground

• `packages/playground/vite.config.ts` — conditional `base`:
  • Dev (`command === 'serve'`): base `/`, so the existing Playwright suite at `localhost:5173/` is unaffected.
  • Build: base `/playground/`, so when the SPA is hosted at `tabmesh.dev/playground/`, its assets resolve to `tabmesh.dev/playground/assets/...` instead of root `/assets/`.

Build orchestration

• New root script `build:site` chains:
  1. `pnpm --filter '@tabmesh/playground^...' build` — builds core, react, transport-websocket
  2. `pnpm --filter @tabmesh/playground build` — builds the playground SPA (writes to `packages/playground/dist/`)
  3. `pnpm docs:build` — builds VitePress (writes to `docs/.vitepress/dist/`)
  4. `cp -R packages/playground/dist/. docs/.vitepress/dist/playground/` — merges the playground into the docs output

Vercel config

• `vercel.json` — build command, install command, output directory `docs/.vitepress/dist`, cleanUrls. No framework preset because the build chain is custom.

How it routes

• `tabmesh.dev/` → VitePress landing (`docs/index.md`)
• `tabmesh.dev/guide/...` etc. → VitePress pages
• `tabmesh.dev/playground` → VitePress page that iframes `/playground/index.html`
• `tabmesh.dev/playground/index.html` → playground SPA, assets at `/playground/assets/...`

Test plan

• `pnpm build:site` succeeds locally
• `docs/.vitepress/dist/playground/index.html` exists with `/playground/`-prefixed asset URLs
• `pnpm exec vitest --run` — 122/122 pass
• `pnpm exec playwright test --grep "single WS"` — passes (dev base unchanged)
• Biome clean

What's still needed before tabmesh.dev is live

• Domain purchase + DNS pointing at Vercel (user)
• Vercel project creation pointing at this repo (user — `vercel.json` picks up the rest)