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.

frontend/index.html

@@ -3389,7 +3389,7 @@ function onSearchChange() {
 }
 
 // ──────────────────── Versioning ────────────────────
-const PWA_VERSION = '2.7.9';
+const PWA_VERSION = '2.7.10';
 const MIN_SERVER_VERSION = '2.3.0';
 
 function parseVersion(v) {