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/models/setting.ts

@@ -103,6 +103,9 @@ export class AllSetting {
   hasWarpSecret = false;
   hasNordSecret = false;
   hasSmtpPassword = false;
+  clearTgBotToken = false;
+  clearLdapPassword = false;
+  clearSmtpPassword = false;
 
   constructor(data?: unknown) {
     if (data != null) {