fix(exports): configure package exports and npm distribution (#568)

[extolkom]

Summary

Closes #568

package.json declared "private": false (intended for npm publishing) but
was missing main, module, exports, and files fields. Without files,
npm publish would have included src/, dev config, and other repo internals
in the tarball. Without main/module/exports, consumers would get no
working entry point.

Changes

• src/index.ts: new root library entry point re-exporting the public API
• package.json: added main, module, types, exports (with types/
  import/require conditions), and files: ["dist", "README.md", "CHANGELOG.md"]
• vite.config.ts / vite.lib.config.ts: library build mode outputting
  dist/index.js (ESM), dist/index.cjs (CJS), dist/style.css, with the
  dts plugin generating type declarations
• tsconfig.lib.json: scoped TypeScript config for the declaration build

Verification — INCOMPLETE, do not merge yet

• npm run build — succeeds and produces the expected dist/ output
  (index.js, index.cjs, index.d.ts, style.css) — not yet confirmed with
  real output pasted here
• npm pack --dry-run — tarball file list contains only dist/,
  README.md, CHANGELOG.md — not yet confirmed
• Real consumer smoke test: pack + install into a scratch project,
  confirm both import (ESM) and require (CJS) resolve and work, and
  TypeScript sees correct types with no missing-declaration error — not
  yet run
• Existing test suite and lint — not yet confirmed clean after this change
• size-limit check (added in this change) — not yet confirmed passing
• CI workflow steps replicated locally exactly — not yet confirmed

Acceptance criteria

• package.json has main, module, exports, and files fields — implemented,
  pending pack verification above
• npm pack produces a tarball containing only dist/, README.md, CHANGELOG.md
• Library build outputs to dist/
• Consumer importing sorokit-ui gets the ESM build by default
• TypeScript types exported alongside the build output

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