fix(settings): surface API error envelopes in the storage tab instead of crashing (#14846)

[shipsfromrio]

Summary

In Settings > Storage, "Backup now" and "Import Database" crash the whole settings page into its error boundary ("Failed to load settings"). "Export Database" shows Export failed: [object Object].

SystemStorageTab used data.error || t(...) as the status message. When the API answers with the structured envelope { error: { code, message, correlation_id } }, that object is rendered as a React child ("Objects are not valid as a React child"), which triggers settings/error.tsx. The download helper did new Error(data.error || fallback), which turns the object into [object Object].

/api/db-backups* has been in ALWAYS_PROTECTED_API_PATHS since before v3.8.50 (the reporter's version). A dashboard session without auth therefore gets exactly this envelope from the authz pipeline (rejectionResponse in src/server/authz/pipeline.ts). The route handlers also return object-shaped error values for validation failures (restore, retention, cleanup).

Every error-status message in the tab, and fetchAndDownload, now goes through the existing extractApiErrorMessage() (src/shared/http/apiErrorMessage.ts), the helper already used for #5340 and #5991. Plain string errors from the route handlers are shown as before.

Related Issues

Closes #14846

Validation

• npx vitest run tests/unit/ui/system-storage-backup-auth-envelope-14846.test.tsx: 2/2 pass. With only the component reverted to the base version, both fail with Objects are not valid as a React child (found: object with keys {code, message, correlation_id}) and exportFailedWithError: [object Object].
• node --import tsx/esm --test tests/unit/ui/system-storage-error-envelope-14846.test.ts: 5/5 pass, 3 fail on the base version.
• Existing tests still pass: npx vitest run tests/unit/ui/system-storage-manual-vacuum.test.tsx tests/unit/ui/system-storage-tab-guest-401-12709.test.tsx, and node --import tsx/esm --test tests/unit/dashboard-localization-contract.test.ts tests/unit/quota-card-grid-mobile-7072.test.ts tests/unit/settings-ui-layout-static.test.ts tests/unit/api-error-message-5340.test.ts (27/27).
• npx eslint and npx prettier --check are clean on the touched files.

Tests Added

• tests/unit/ui/system-storage-backup-auth-envelope-14846.test.tsx: renders the tab with a 401 AUTH_001 envelope from /api/db-backups and clicks "Backup now" and "Export Database". It checks that the tab stays mounted, that the envelope message is shown, and that [object Object] never appears.
• tests/unit/ui/system-storage-error-envelope-14846.test.ts: a source guard against message: data.error || and new Error(data.error) in the tab, plus extractor checks on the pipeline envelope, string errors and empty bodies.

Coverage Notes

The .sqlite import confirm flow (file input plus modal) is covered by the source guard, not by a render test. It uses the same code path as "Backup now".

Reviewer Notes

• The 401 itself is intended (GHSA-mghq-58h3-qcqj) and is left as is. After this change, an unauthenticated user sees the auth message (and the existing auth-required banner from [BUG] Importing JSON configuration in guest session causes Settings to show "Failed to load settings" #12709) instead of a crashed page or [object Object].
• catch (err) changed to catch {} in the legacy JSON import handler. eslint already flagged that line before this change.
• Feature/delete error modelsfeat(ui): add remove error models button in combo test results, default autoHideFailed, and fix db import crash #14944 partly overlaps: it unwraps the error only in the .sqlite import handler and adds a render-time guard. This PR fixes it at the source for every handler, including the export [object Object] message.