Phase-2 review finding: the validation branch expanduser()s the path but
web_server.py reads os.environ['HERMES_WEB_DIST'] raw at import — a
'~/dist' value would validate here and still 404 there. Write the
expanded path back before the web_server import. Adds a regression test
asserting the env var holds the expanded path after cmd_dashboard.
A custom HERMES_WEB_DIST without --skip-build skipped BOTH the web UI
build and any validation: cmd_dashboard fell through the build gate and
started the server against a dist that may not exist, serving 404s with
no obvious cause. This is the same failure mode issue #23817 fixed for
the --skip-build branch — the env-var branch was left unvalidated.
Add the missing else-branch: fail fast with actionable guidance when
HERMES_WEB_DIST has no index.html, proceed (still without building) when
it does.
Credit: @Caelier (#17845) originally proposed dist validation for the
dashboard startup path; the --skip-build half of that PR's scope has
since landed via the #23817 fix, this covers the remaining env-var path
on the rewritten cmd_dashboard surface.
