Clarify .exe suffix handling in class normalization

internal/winapi/class.go

@@ -14,8 +14,9 @@ func ClassFromImagePath(p string) string {
 	if i := strings.LastIndexAny(p, `\/`); i >= 0 {
 		p = p[i+1:]
 	}
-	if len(p) >= 4 && strings.EqualFold(p[len(p)-4:], ".exe") {
-		p = p[:len(p)-4]
+	const exeSuffix = ".exe"
+	if len(p) >= len(exeSuffix) && strings.EqualFold(p[len(p)-len(exeSuffix):], exeSuffix) {
+		p = p[:len(p)-len(exeSuffix)]
 	}
 	return strings.ToLower(p)
 }

internal/winapi/class_test.go

@@ -12,6 +12,7 @@ func TestClassFromImagePath(t *testing.T) {
 		{`C:/forward/slash/Code.exe`, "code"},
 		{`firefox`, "firefox"},
 		{``, ""},
+		{`.exe`, ""}, // degenerate: only the extension
 	}
 	for _, c := range cases {
 		if got := ClassFromImagePath(c.in); got != c.want {