bug(admin/policies): non-admin roles can edit Purchasing Policies fields; save fails with 'admin access required' (regression after #870/PR #877)

[cristim]

Repro

1. Log in as any non-admin user (Viewer, Standard, Plan Authors, Purchase Approver, or Read-Only).
2. Navigate to Admin > Purchasing Policies.
3. Try clicking on any value field (e.g., Term, Payment, Target Coverage).
4. Change the value and click 'Save Settings'.

Expected

All Purchasing Policies input fields are read-only (disabled/non-interactive) for non-admin users. The Save and Reset buttons are hidden or disabled. The user can only view the current settings.

Actual

The value fields are clickable and accept input. When the user attempts to save, the error message 'Failed to save settings: admin access required' appears. The fields should never have been editable in the first place.

Context

Issue #870 and PR #877 fixed this for the Viewer role. The fix has regressed and now affects all non-admin roles. QA confirmed: 'The fields are clickable again (used to be fixed).'

Affects: Viewer (Admin tab row 6.2, Viewer User Role tab row 6.2), Plan Authors (Plan Authors tab row 7.2), Standard user (Standard User Role tab row 7.2), Purchase Approver (Purchase Approver tab row 7.2), Bugs tab row 493.

Sheet reference

Bugs and improvement suggestions tab, row 493. Viewer User Role tab, row 6.2. Plan Authors User Role tab, row 7.2. Standard User Role tab, row 7.2. Purchase Approver User Role tab, row 7.2.