Packaging: the wheel carries the PWA, so a release install serves the UI (v2.7.10)

- pyproject declared build-backend = "setuptools.backends._legacy:_Backend",
  which does not exist: `python -m build`, `pip install .` and even
  `pip install -e .` died with "Backend ... is not available".  It is
  setuptools.build_meta now.
- the wheel and sdist carried only the Python packages, and _default_web_dir()
  resolved <site-packages>/../frontend, so an installed server started fine but
  answered every static request with 404.  setup.py's build_py now copies the
  checkout's frontend/ into simtrace_analyser/web/ inside the build (the PWA's
  tests excluded), and _default_web_dir() prefers that packaged copy, falling
  back to <repo>/frontend for source and editable runs.  The source directory
  is resolved from setup.py (so an sdist build works) and a missing frontend/
  fails the build loudly instead of shipping a package without its UI.
- MANIFEST.in grafts frontend/ (with the READMEs, the udev rule and the four
  launch scripts) into the sdist, so a wheel built from the sdist has the PWA.
- README (EN/RU): an install-from-a-release section (pipx/pip with the wheel,
  the direct release URL for pip) and the --web-dir default documented as the
  bundled PWA, or <repo>/frontend from source.
- tests/test_packaging.py pins the packaged-first resolution (including the
  empty-directory fallback), the source checkout default, the real build
  backend, the manifest graft and setup.py's bundling.
- verified: a fresh venv install from the wheel and from the sdist serve
  GET / (200, byte-identical index.html, PWA v2.7.10) out of
  site-packages/simtrace_analyser/web; an editable install still resolves
  <repo>/frontend.  547 Python + 80 Node tests.

simtrace_analyser/__main__.py

@@ -16,10 +16,20 @@ from .capture import CaptureManager, GsmtapListener, DirectSniffer
 from .version import __version__
 
 
+def _pick_web_dir(package_dir, repo_root):
+    """The installed package's bundled PWA first, then the source checkout."""
+    packaged = os.path.join(package_dir, 'web')
+    if os.path.isfile(os.path.join(packaged, 'index.html')):
+        return packaged
+    return os.path.join(repo_root, 'frontend')
+
+
 def _default_web_dir():
-    # <repo>/frontend, whether run from source or an editable install.
-    # __file__ is <repo>/simtrace_analyser/__main__.py → up two levels.
-    return os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))), 'frontend')
+    # A wheel/sdist install carries simtrace_analyser/web (copied there by
+    # setup.py); an editable install and a plain source run fall through to
+    # <repo>/frontend.  __file__ is <repo>/simtrace_analyser/__main__.py.
+    pkg_dir = os.path.dirname(os.path.abspath(__file__))
+    return _pick_web_dir(pkg_dir, os.path.dirname(pkg_dir))
 
 
 def main():
@@ -42,7 +52,9 @@ def main():
     p.add_argument('--log-requests', action='store_true', default=False,
                    help='Log every HTTP request (off by default)')
     p.add_argument('--web-dir', default=_default_web_dir(), metavar='PATH',
-                   help='Directory with the simtrace-analyser PWA static files to serve (default: <repo>/frontend)')
+                   help='Directory with the simtrace-analyser PWA static files to serve '
+                        '(default: the PWA bundled in the installed package, '
+                        'or <repo>/frontend in a source checkout)')
     args = p.parse_args()
 
     db = Database(args.db)