feat(email): PWA login sends code only; drop admin link from alert emails

Two email changes, per the owner:

1. Code-only login from an installed PWA. A login requested while running as a
   standalone PWA now omits the one-tap confirm link from the email — the link
   would open in a separate browser whose minted session cannot reach the PWA,
   stranding the login. The code is typed in the same window instead. The client
   sends a `pwa` flag (isStandalone) on the email-code request (a new FBS field,
   threaded through the gateway); the backend omits the deeplink when it is set,
   reusing the existing deletion-code link-omission path. Non-PWA browser logins
   keep the one-tap link. No polling, no migration.

2. Security: the operator alert digest no longer embeds the admin-console (/_gm)
   URL. An admin link must never travel in an email, where a mail provider could
   cache or index it; the operator opens the console directly.

Tests: an inttest asserting the PWA login email omits the link (and a browser one
keeps it); a codec round-trip of the new pwa field; the alert-digest test flipped
to guard the admin link is absent. Docs: ARCHITECTURE + FUNCTIONAL (+ru).

backend/internal/account/link.go

@@ -92,7 +92,7 @@ func (s *EmailService) RequestLinkCode(ctx context.Context, accountID uuid.UUID,
 	if !s.allowSend(addr) {
 		return ErrTooManyRequests
 	}
-	return s.issueCode(ctx, accountID, addr, purposeLink, s.accountLocale(ctx, accountID))
+	return s.issueCode(ctx, accountID, addr, purposeLink, s.accountLocale(ctx, accountID), false)
 }
 
 // ConfirmLink verifies code for (accountID, email) and reports the address's
@@ -140,7 +140,7 @@ func (s *EmailService) RequestChangeCode(ctx context.Context, accountID uuid.UUI
 	if !s.allowSend(addr) {
 		return ErrTooManyRequests
 	}
-	return s.issueCode(ctx, accountID, addr, purposeChange, s.accountLocale(ctx, accountID))
+	return s.issueCode(ctx, accountID, addr, purposeChange, s.accountLocale(ctx, accountID), false)
 }
 
 // ConfirmChange verifies code for (accountID, newEmail) and atomically replaces the
@@ -199,7 +199,7 @@ func (s *EmailService) RequestDeleteCode(ctx context.Context, accountID uuid.UUI
 	if !s.allowSend(addr) {
 		return ErrTooManyRequests
 	}
-	return s.issueCode(ctx, accountID, addr, purposeDelete, s.accountLocale(ctx, accountID))
+	return s.issueCode(ctx, accountID, addr, purposeDelete, s.accountLocale(ctx, accountID), false)
 }
 
 // VerifyDeleteCode verifies the account-deletion code against the account's own email and