fix(explorer): resolve blank dashboard UI and ship frontend bundle in wheel (#638)

* fix(explorer): resolve blank dashboard UI and ship frontend bundle in wheel

Fixes #631 — the Explorer server started successfully but the browser showed
a blank page because semantica/static/ was gitignored and never present after
a fresh install or clone.

Changes:
- ci.yml / release.yml: add Node 20 setup + npm ci && npm run build before
  python -m build so every wheel contains a CI-built frontend bundle
- pyproject.toml: add package-data patterns (static/*, static/assets/*) so
  setuptools includes the bundle in the wheel; add MANIFEST.in for sdist coverage
- app.py: replace silent empty-HTML fallback with a 200 page that clearly
  explains the missing bundle and links to /docs; fix CORS allow_credentials
  to default false, gated behind EXPLORER_CORS_CREDENTIALS env var to prevent
  credentialed cross-origin requests on unauthenticated endpoints
- __init__.py: warn at startup when --host is non-loopback (unauthenticated
  network exposure)
- explorer/README.md: full rewrite covering pip-install mode (primary path,
  no Node required) and dev-server mode (contributors), CLI flags, env vars,
  workspace table, troubleshooting for the blank-page symptom
- README.md: update Knowledge Explorer section with correct command and link
  to the new setup guide

* fix(explorer): set build.target esnext to fix esbuild CI failure

esbuild >=0.28 (forced via npm overrides) conflicts with Vite 6 defaults on
Linux CI — it tries to lower destructuring syntax for the implicit browser
target list but errors out. Explicit target: 'esnext' tells esbuild to emit
native syntax unchanged, bypassing the transpilation error entirely. Safe for
a developer tool that runs in modern browsers.

* test(explorer): verify packaged frontend bundle

---------

Co-authored-by: Zohaib Hassnain <109234410+ZohaibHassan16@users.noreply.github.com>

MANIFEST.in

@@ -0,0 +1 @@
+recursive-include semantica/static *