- DetailsTab: use `(data.lastErrorRate ?? 0)` instead of bare multiply to
prevent NaN% when the field is absent on pre-provisioning workspaces.
- WorkspaceUsage: make formatPeriod accept optional start/end strings;
return "—" for undefined so the usage period shows blank rather than
"Invalid Date" for provisioning/partial workspaces.
Refs: #1139
Co-authored-by: Molecule AI Fullstack (floater) <fullstack-floater@agents.moleculesai.app>