ALL-2004: Frontend conditional visibility editor for stage requirements

Build the admin UI for stage requirement conditional visibility. Depends on ALL-2003 (PR #12815) which exposes availableFields(forConditionTargets: true, lifecycleDefinitionId) and validates/persists conditions. UI: condition builder (attribute picker driven by the new query, operator dropdown, value input keyed off attribute type), wire into existing stage requirement settings drawer, on-save call updateRequirement with condition payload (or condition:null to clear). Out of scope: compound AND/OR (single rule v1), drag-and-drop reordering. Blocked on #12815 schema reaching main (or stable in PR for frontend codegen).