refactor: the server says a count one way, words.Count, for notifications, a wanted match's reason and the duplicate resolver's activity line (M505 #5643)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/words/words.go

@@ -0,0 +1,15 @@
+// Package words says counts in English for what the server writes to people:
+// notifications, emails, the reasons a scan or a match gives.
+package words
+
+import "strconv"
+
+// Count says n with the noun that agrees with it: "1 track", "3 tracks". The
+// noun may carry its verb ("track went missing" / "tracks went missing").
+// The web app's plural() is the same thing on the other side (M505).
+func Count[N ~int | ~int32 | ~int64](n N, one, many string) string {
+	if n == 1 {
+		return "1 " + one
+	}
+	return strconv.FormatInt(int64(n), 10) + " " + many
+}

internal/words/words_test.go

@@ -0,0 +1,19 @@
+package words
+
+import "testing"
+
+func TestCount(t *testing.T) {
+	cases := []struct {
+		got, want string
+	}{
+		{Count(1, "track", "tracks"), "1 track"},
+		{Count(0, "track", "tracks"), "0 tracks"},
+		{Count(int64(3), "track went missing", "tracks went missing"), "3 tracks went missing"},
+		{Count(int32(-1), "copy", "copies"), "-1 copies"},
+	}
+	for _, c := range cases {
+		if c.got != c.want {
+			t.Errorf("got %q, want %q", c.got, c.want)
+		}
+	}
+}