Move to Go 1.27, and fix initials for non-ASCII names

Initials() capped the loop with len(out) == 2, which counts bytes: a name
starting with Ä, Ö or Å filled the budget on its own and returned a single
letter. Count the initials taken instead.

go fix also wanted a strings.Builder here, but that allocates a string per
iteration to measure a two-character result. Took its SplitSeq suggestion
in lyrics.go, which drops an intermediate slice.

auth_test.go

@@ -230,3 +230,17 @@ func TestBanDropsSessionsAndBlocksLogin(t *testing.T) {
 		t.Fatalf("login after unban: status = %d, want 303", w.Code)
 	}
 }
+
+func TestInitials(t *testing.T) {
+	for name, want := range map[string]string{
+		"Esa Kataja":  "EK",
+		"Ärväs Öhman": "ÄÖ", // multi-byte initials must not end the loop early
+		"Åke":         "Å",
+		"":            "",
+		"a b c":       "AB",
+	} {
+		if got := (&member{Name: name}).Initials(); got != want {
+			t.Errorf("Initials(%q) = %q, want %q", name, got, want)
+		}
+	}
+}