UI/UX improvements: searchable linkage, tooltips, workflow stepper

- Add Tooltip component (keyboard/AT accessible) and FormField `tip` prop
- Add SearchSelect async combobox (loading/empty/error, ARIA combobox)
- Add WorkflowStepper component for risk/workflow stages
- Report -> hazard linkage now uses SearchSelect instead of raw hazard_id
- Risk -> linked-items uses SearchSelect via ReferenceDataController::searchEntities
- Disposition picker converted to radio-cards with descriptions
- Risk workflow rendered as stepper with per-stage tooltips
- Status/band badges gain explanatory tooltips
- Extend Guide page with risk workflow / acceptance / effectiveness sections
- Add doc/UI_UX_IMPROVEMENT_PLAN.md and screenshot assets
