Improve validation

Tests macOS/InstancesModelTests.swift

@@ -0,0 +1,17 @@
+import XCTest
+
+final class InstancesModelTests: XCTestCase {
+    func testStandardizedURL() throws {
+        let samples: [String: String] = [
+            "https://www.youtube.com/": "https://www.youtube.com",
+            "https://www.youtube.com": "https://www.youtube.com",
+        ]
+
+        samples.forEach { url, standardized in
+            XCTAssertEqual(
+                InstancesModel.standardizedURL(url),
+                standardized
+            )
+        }
+    }
+}