Allow workflow recent document windows to use minutes, hours, or days

## Summary
Workflow recent-document search windows currently appear to use a fixed minutes unit. Recent documents should allow the window unit to be selected as minutes, hours, or days so workflow authors can choose the appropriate recency range.

## User Value
Workflow authors can tune recent-document context retrieval for short, medium, or longer lookback windows without needing awkward minute conversions or being limited to very short windows.

## Proposed Behavior
For workflow actions that target recent documents, the Recent Window unit selector should support:

- Minutes
- Hours
- Days

The selected unit should be persisted with the workflow configuration and honored when retrieving recent documents for workflow context.

## Acceptance Criteria
- [ ] Recent-document workflow configuration exposes a selectable unit for minutes, hours, and days.
- [ ] The selected unit is saved and restored when editing an existing workflow.
- [ ] Recent-document retrieval uses the selected unit when calculating the lookback window.
- [ ] Validation prevents invalid or unsupported window units.
- [ ] Functional or UI coverage verifies minutes, hours, and days behavior.

## Notes
This applies to workflow actions using recent documents as the document target. The current UI shows the Recent Window with a fixed minutes suffix.