feat(settings): let users clear stored secrets from the UI

Redacted secrets (SMTP password, Telegram bot token, LDAP password) are
always served blank to the browser, so the update path treats a blank
submission as "unchanged" and silently restores the stored value. That
made a once-set secret impossible to remove without editing the database
— e.g. switching to a passwordless localhost SMTP relay kept sending the
old credentials forever.

Blank stays "unchanged"; clearing is now its own signal. The update
request carries explicit clear flags (request-scoped fields on the
controller form, so they are never persisted as settings rows), and
preserveRedactedSecrets skips the restore for a flagged secret. Each
secret field gets a Clear/Undo button that arms the flag; typing a new
value disarms it. The 2FA token keeps its existing behavior: it is
already clearable by disabling 2FA.

Closes #5724

frontend/src/pages/settings/SecretInput.tsx

@@ -0,0 +1,45 @@
+import { Button, Input, Space } from 'antd';
+import { useTranslation } from 'react-i18next';
+
+interface SecretInputProps {
+  value: string;
+  configured: boolean;
+  clearArmed: boolean;
+  placeholder: string;
+  onChange: (value: string) => void;
+  onClearArmedChange: (armed: boolean) => void;
+}
+
+export default function SecretInput({
+  value,
+  configured,
+  clearArmed,
+  placeholder,
+  onChange,
+  onClearArmedChange,
+}: SecretInputProps) {
+  const { t } = useTranslation();
+  return (
+    <Space.Compact style={{ width: '100%' }}>
+      <Input.Password
+        value={value}
+        placeholder={configured && !clearArmed ? placeholder : ''}
+        onChange={(e) => {
+          onChange(e.target.value);
+          if (clearArmed) onClearArmedChange(false);
+        }}
+      />
+      {configured && (
+        <Button
+          danger={clearArmed}
+          onClick={() => {
+            onChange('');
+            onClearArmedChange(!clearArmed);
+          }}
+        >
+          {clearArmed ? t('pages.settings.secretClearUndo') : t('pages.settings.secretClear')}
+        </Button>
+      )}
+    </Space.Compact>
+  );
+}