fix(quick-261029-d9j): pick the mail language from Accept-Language, add mail.fallback_locale, Polish subjects

- preferred_locale and the request locale are normalised (comma lists,
  q-weights, region subtags, case) to pl or en by NormalizeMailLocale, so a
  raw Accept-Language such as en-GB,en;q=0.9 selects the -en templates
- new golem15.user.mail.fallback_locale (default pl) is used when neither
  names a supported language; ResolveMailLocaleFor applies it, and Boot
  rejects any value other than pl or en
- activate, reactivate and restore Polish templates get Polish subjects
- no template, route or payload changes; MailTemplate and
  ResolveMailLocale keep their signatures

mail_subjects_test.go

@@ -0,0 +1,58 @@
+package user
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"git.golem15.com/golem15/summercms/modules/postcard"
+)
+
+// TestPolishMailSubjects renders every Polish template next to its -en twin
+// and requires a subject of its own, so a Polish template with a copied
+// English subject does not slip through.
+func TestPolishMailSubjects(t *testing.T) {
+	p := &Plugin{}
+	cat := postcard.NewCatalog()
+	if err := cat.Register(p.ID(), p.MailTemplatesFS(), p.MailTemplates(), p.MailLayouts()); err != nil {
+		t.Fatal(err)
+	}
+	names := map[string]bool{}
+	for _, name := range p.MailTemplates() {
+		names[name] = true
+	}
+	vars := map[string]any{
+		"name": "Ada", "Name": "Ada",
+		"link": "https://example.com/link", "Link": "https://example.com/link",
+		"code": "1!abc", "Code": "1!abc",
+		"hours": 72, "Hours": 72,
+		"login": "ada@example.test", "Login": "ada@example.test",
+	}
+	subject := func(t *testing.T, template string) string {
+		t.Helper()
+		mem := postcard.NewMemoryDriver()
+		mailer := postcard.NewMailer(cat, mem, postcard.Options{From: "noreply@example.test"})
+		if err := mailer.Send(context.Background(), postcard.Message{Template: template, To: []string{"ada@example.test"}, Vars: vars}); err != nil {
+			t.Fatalf("%s: %v", template, err)
+		}
+		msgs := mem.Messages()
+		if len(msgs) != 1 {
+			t.Fatalf("%s: %d messages", template, len(msgs))
+		}
+		return msgs[0].Subject
+	}
+	pairs := 0
+	for _, name := range p.MailTemplates() {
+		if strings.HasSuffix(name, "-en") || !names[name+"-en"] {
+			continue
+		}
+		pairs++
+		pl, en := subject(t, name), subject(t, name+"-en")
+		if pl == "" || pl == en {
+			t.Errorf("%s: Polish subject %q, English subject %q", name, pl, en)
+		}
+	}
+	if pairs == 0 {
+		t.Fatal("no Polish/English template pairs")
+	}
+}