ci: coverage reports for the native and web UI tests

A coverage env (the native env with gcov counters) runs the native
tests in CI; gcovr writes the HTML report and a table on the run's
summary page. The web UI tests report coverage with Node's built-in
coverage, and genhtml turns its lcov into HTML. Both reports are run
artifacts, and pushes to main publish them to GitHub Pages.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

platformio.ini

@@ -161,3 +161,14 @@ build_flags =
 	-Isrc
 	-Itest/stubs
 	-D_POSIX_THREAD_SAFE_FUNCTIONS
+
+; The native tests with gcov counters, for the coverage report:
+;   pio test -e coverage && gcovr
+; gcovr.cfg sets what the report covers and where it goes.
+[env:coverage]
+extends = env:native
+build_flags =
+	${env:native.build_flags}
+	--coverage
+	-O0
+	-lgcov