Per-field Save buttons were an unconventional, endpoint-driven choice. Replace
them with a single drafted Save/Discard bar (consistent with the Settings page):
edits are buffered and applied together on Save (each via the existing per-key
PATCH/DELETE), Discard reverts. Reset-to-default is a per-field affordance applied
on Save too — clearing a non-secret falls back to default; a secret (which can't
be cleared by emptying) gets a toggle that resets it on save. Backend unchanged.
New admin-only Configuration page that renders the system_config registry grouped
(Email/SMTP first), with per-field save/reset, write-only encrypted secret fields
(disabled with a hint when TOKEN_ENCRYPTION_KEY is unset), and a Send-test-email
button. New 'config' page + sidebar nav item + header title; api methods and
EN/HU/DE strings.
