Add local demo app for previewing the builder

[zhawtof]

Summary

• Adds a Vite + React demo app under demo/ so the library can be exercised in a browser without publishing or wiring up the full template repo.
• The demo imports BlockKitBuilder directly from src/index.ts via a Vite alias — edits to library source hot-reload in the demo.
• Mocks loadChannels, loadSendAsUserStatus, and onSend so the palette → surface → send-dialog flow works end-to-end without a backend.
• Adds .claude/launch.json so the Claude Code preview tool can boot the server on port 5173.

Why

There was no first-party way to render the component locally — the closest thing is the external block-kit-builder-template repo, which couples preview to a full Cloudflare Workers + slack-hono backend. That's heavy for iterating on the builder itself.

This adds a minimal in-tree harness for fast visual iteration, similar in spirit to a Storybook setup but cheaper.

Notes for reviewers

• The demo is a separate package (demo/package.json) with its own node_modules so library devDependencies don't grow.
• The Vite alias means library source changes hot-reload; you do not need to run pnpm build in the root to see updates.
• Demo styles include a default shadcn token set (--background, --primary, etc.) since the library only declares fallbacks.
• demo/pnpm-workspace.yaml was generated by pnpm approve-builds esbuild and is needed so a fresh pnpm install in demo/ doesn't error on pnpm 11's ignored-builds check.

Test plan

• cd demo && pnpm install succeeds on a clean checkout
• pnpm --dir demo dev starts on port 5173
• Builder renders with the seeded header/section/divider blocks
• Drag a block from the palette into the surface — appears in preview
• Click Send → dialog lists mock channels → submit → browser alert fires and payload logs to console
• Toggle the demo page theme button → preview chrome flips light/dark

🤖 Generated with Claude Code