bug(ui): Retention tab never renders the backup status the API already returns #3454

Summary

The response carries a backup block; the tab shows nothing about backups.

Category: functional · Severity: Medium · Validated: API

Context

Found by the click-through UI sweep of a local instance (dev @ c90c711ee, 1.0.0-rc1), fixture agents only.

Repro

1. Open /settings?tab=retention.
2. Compare with GET /api/settings/retention.

Expected

Honest status: the automatic-backup state (last run, recovery points) is visible to the operator.

Observed

The response carries a backup block; the tab shows nothing about backups.

Validation

GET /api/settings/retention returns a backup key; no "backup" string exists in src/frontend/src/views/Settings.vue or src/frontend/src/components/settings/.

Acceptance Criteria

• The repro above gives the Expected result at 1440 and 768, in light and dark.
• A regression test pins the behaviour (unit or Playwright, whichever tier can observe it).
• No new raw-palette classes or hand-rolled primitives are introduced by the fix (docs/memory/design-system-contract.md).

Technical Notes

Evidence screenshots (kept on the machine that ran the sweep; the evidence folder is gitignored because the repository is public):

• docs/testing/ui-sweep/evidence/2026-10-09-all/8c213903-1.png

Found by /ui-sweep --all @ c90c711