fix(tests): skip web-bundle assertion when the frontend is not built

[rajathpi]

Problem

On a fresh clone, pip install -e '.[test]' && pytest fails with one error:

FAILED tests/webapi/test_webapi.py::test_legacy_static_dashboard_routes_are_not_registered
assert 404 == 200

Cause

The test asserts that GET / serves the SPA, but the web bundle lives at src/lh_harness/_frontend/web/dist, which is git-ignored and only produced by npm run build --prefix frontend/web. create_app already tolerates the missing bundle (server.py: if _STATIC_DIR.is_dir(): app.mount(...)) — packaging deliberately treats it as a build artifact so a checkout without a Node toolchain still installs — but the test did not. CI never hits this because release.yml builds the frontend before running pytest, and no workflow runs on pull_request, so nothing catches it for contributors.

Fix

• The legacy-route assertions (what the test's name is about) need no bundle, so they now always run.
• The SPA-serving assertions move to a new test, test_built_web_bundle_is_served_at_root, guarded by @pytest.mark.skipif(not _STATIC_DIR.is_dir(), ...) with an actionable skip reason. The condition mirrors create_app's own guard, so the test and production behavior cannot drift.

Verification

• Without the bundle: 169 passed, 1 skipped, with the skip reason printed under -rs.
• With the bundle (CI-equivalent, after npm ci --prefix frontend/web --no-audit --no-fund && npm run build --prefix frontend/web): 170 passed — the new test genuinely runs when the bundle exists.