New WorkflowOptionalStepsPanel (sibling to Fields/Settings) lets authors
add/remove optional steps and set each one's defaultOn, with unknown ids
shown as muted removable rows. Wire optionalSteps state through both load
paths (incl. the fragment path, which also dropped settings), every
flowToIr/serializeGraph call site, and the save handler deps — fixing a
stale-closure that dropped defaultOn edits on save. Extract the shared
phaseBadge helper. Mobile gets an Optional steps tab too.
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