phishing alert implementation

we now check when building the string through the `AttributedStringBuilder` if a URL is actually hiding a different link, if so, we create a custom URL that contains both the external and the internal URL to advise the user through an Alert about the risk

ElementX/Sources/Other/Extensions/CharacterSet.swift

@@ -0,0 +1,31 @@
+//
+// Copyright 2025 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Foundation
+
+extension CharacterSet {
+    private static let urlAllowedSets: [CharacterSet] = [
+        .urlUserAllowed,
+        .urlPasswordAllowed,
+        .urlHostAllowed,
+        .urlPathAllowed,
+        .urlQueryAllowed,
+        .urlFragmentAllowed
+    ]
+    
+    static let urlAllowedCharacters: CharacterSet = {
+        // Start by including hash, which isn't in any URL set
+        // Then include all URL-legal characters
+        var result = CharacterSet(charactersIn: "#")
+        for set in urlAllowedSets {
+            result.formUnion(set)
+        }
+        return result
+    }()
+    
+    static let matrixUserIDAllowedCharacters = CharacterSet(charactersIn: "abcdefghijklmnopqrstuvwxyz0123456789._=-/@:")
+}