refactor(config): one Save/Discard bar instead of per-field Save

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.

frontend/src/i18n/locales/en/config.json

@@ -12,8 +12,12 @@
     "smtp_password": { "label": "SMTP password", "hint": "App password. Stored encrypted; write-only — it's never shown back." }
   },
   "save": "Save",
+  "saving": "Saving…",
   "saved": "Saved",
   "saveFailed": "Couldn't save",
+  "unsaved": "Unsaved changes",
+  "discard": "Discard",
+  "willReset": "will reset to default on save",
   "reset": "Reset",
   "resetHint": "Remove this override and fall back to the file/env default.",
   "resetDone": "Reset to default",