improvement(data-retention): convert policy editor from modal to full-surface page

Mirror the access-control group-detail pattern: clicking a retention policy
now drills into a full-surface PolicyDetail page (back chip + dirty-gated
header Save/Discard + Remove) instead of an xl modal. Form fields use
SettingsSection (Workspaces / Retention / PII redaction) and the page chrome
matches group-detail exactly. The unsaved-changes confirm stays a small modal.

Author: waleedlatif1