feat(admin-ui): add 2PC settings preview endpoint and UI integration
Implements P5.19.b §13.19 - Indexes + Aliases sections with LIVE 2PC preview.
Backend changes:
- Add POST /indexes/{index}/settings preview endpoint
- Returns current vs proposed settings with SHA256 fingerprints
- Shows node targets, version info, and diff summary
- Displays full two-phase flow (propose/verify/commit) details
- Export compute_settings_diff for testing
Frontend changes:
- Update previewSettingsChanges() to call new preview endpoint
- Display current/proposed fingerprints, version info
- Show node targets and two-phase flow steps
- Render structured diff (added/removed/modified)
Tests:
- Add p13_19_admin_ui_2pc_preview.rs acceptance tests
- Verify fingerprint computation, diff detection, node targets
Closes: miroir-uhj.19.2
9d29d757c7