Follow-up to PR #23824. Adds two correctness fixes on top of the
contributor's salvaged commit:
1. Stale-dist fallback no longer gated on `fatal=False`. `cmd_dashboard`
passes `fatal=True` and is the primary scenario this fallback is for
(issue #23817 — Windows Scheduled Task at logon). The previous gate
meant the fallback never fired in the case it was designed for.
2. `--skip-build` now verifies the dist actually exists before starting
the server. Without this, a misconfigured pre-build would launch the
dashboard pointing at a missing dist and silently serve 404s. We now
exit 1 with a clear "pre-build first: cd web && npm run build"
message, and on success print which dist directory is being used.
Verified end-to-end on Linux:
- build fails + stale dist (fatal=True) -> fallback fires
- build fails + no dist (fatal=True) -> exit 1 with stderr surfaced
- build fails + stale dist (fatal=False) -> fallback fires
- --skip-build + missing dist -> exit 1 with clear guidance
- --skip-build + valid dist -> 'Skipping web UI build...'
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