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/cmd/backend/main.go

@@ -12,7 +12,6 @@ import (
 	"fmt"
 	"log"
 	"os/signal"
-	"strings"
 	"syscall"
 	"time"
 
@@ -216,11 +215,9 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	// Operator alert emails on new feedback / word complaints, coalesced into one digest
 	// per interval. Inert unless a distinct admin sender and recipient are configured.
 	if cfg.SMTP.AdminFrom != "" && cfg.SMTP.AdminTo != "" {
-		consoleURL := ""
-		if cfg.PublicBaseURL != "" {
-			consoleURL = strings.TrimRight(cfg.PublicBaseURL, "/") + "/_gm"
-		}
-		alerts := adminalert.New(mailer, feedbackSvc, games, cfg.SMTP.AdminFrom, cfg.SMTP.AdminTo, consoleURL, logger)
+		// The alert digest carries no admin-console link on purpose — an admin URL must not
+		// travel in an email (a mail provider could cache or index it); see adminalert.New.
+		alerts := adminalert.New(mailer, feedbackSvc, games, cfg.SMTP.AdminFrom, cfg.SMTP.AdminTo, logger)
 		go alerts.Run(ctx, adminAlertInterval)
 		logger.Info("admin alert worker started", zap.Duration("interval", adminAlertInterval))
 	}