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/adminalert/notify_test.go

@@ -28,7 +28,7 @@ func (m *recordingMailer) Send(_ context.Context, msg account.Message) error {
 
 func TestNotifierSkipsWhenNothingNew(t *testing.T) {
 	mailer := &recordingMailer{}
-	n := New(mailer, fbCounter{0}, cpCounter{0}, "alerts@erudit-game.ru", "op@x.ru", "", nil)
+	n := New(mailer, fbCounter{0}, cpCounter{0}, "alerts@erudit-game.ru", "op@x.ru", nil)
 	n.tick(context.Background())
 	if len(mailer.sent) != 0 {
 		t.Fatalf("sent %d emails, want 0 when nothing is new", len(mailer.sent))
@@ -37,7 +37,7 @@ func TestNotifierSkipsWhenNothingNew(t *testing.T) {
 
 func TestNotifierDigestsNewItems(t *testing.T) {
 	mailer := &recordingMailer{}
-	n := New(mailer, fbCounter{2}, cpCounter{1}, "alerts@erudit-game.ru", "op@x.ru, two@x.ru", "https://erudit-game.ru/_gm", nil)
+	n := New(mailer, fbCounter{2}, cpCounter{1}, "alerts@erudit-game.ru", "op@x.ru, two@x.ru", nil)
 	n.tick(context.Background())
 	if len(mailer.sent) != 1 {
 		t.Fatalf("sent %d emails, want 1 digest", len(mailer.sent))
@@ -49,7 +49,9 @@ func TestNotifierDigestsNewItems(t *testing.T) {
 	if !strings.Contains(msg.Subject, "2 new feedback") || !strings.Contains(msg.Subject, "1 new word complaint") {
 		t.Errorf("digest subject = %q, want the feedback + complaint counts", msg.Subject)
 	}
-	if !strings.Contains(msg.Text, "/_gm") {
-		t.Errorf("digest body = %q, want the console link", msg.Text)
+	// The digest must never carry an admin-console link — an admin URL in an email is a leak
+	// (mail providers cache/index it).
+	if strings.Contains(msg.Text, "/_gm") || strings.Contains(strings.ToLower(msg.Text), "admin console") {
+		t.Errorf("digest body = %q, must not carry an admin-console link", msg.Text)
 	}
 }