Initial implementation of AccountSettings. (#6275)

* Make the UserPreference macro also generate an accessor for the key name.

* Initial implementation of AccountSettings.

- Add an AccountUserDefaults storage with a userID prefix on the key.
- Migrate the 2 values reset by AppSettings.resetSessionSpecificSettings to AccountSettings.

* Rename UserSettings.volatile() to UserSettings.mock()

Given it now has a userID parameter with a default value..

ElementX/Sources/Application/Settings/Storage/TrackedUserDefaults.swift

@@ -0,0 +1,50 @@
+//
+// Copyright 2026 Element Creations 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
+
+final nonisolated class TrackedUserDefaults: UserDefaults, UserDefaultsProtocol {
+    private let lock = NSLock()
+    
+    private nonisolated(unsafe) var _trackedSuites: [String] = []
+    private(set) var trackedSuites: [String] {
+        get { lock.withLock { _trackedSuites } }
+        set { lock.withLock { _trackedSuites = newValue } }
+    }
+    
+    override init?(suiteName suitename: String?) {
+        if let suitename {
+            lock.lock()
+            _trackedSuites = [suitename]
+            lock.unlock()
+        }
+        
+        super.init(suiteName: suitename)
+    }
+    
+    override func addSuite(named suiteName: String) {
+        trackedSuites.append(suiteName)
+        super.addSuite(named: suiteName)
+    }
+    
+    override func removeSuite(named suiteName: String) {
+        trackedSuites.removeAll { $0 == suiteName }
+        super.removeSuite(named: suiteName)
+    }
+    
+    func resetRegisteredDomains() {
+        lock.withLock {
+            for suite in _trackedSuites {
+                removePersistentDomain(forName: suite)
+            }
+        }
+    }
+    
+    func reset() {
+        resetRegisteredDomains()
+    }
+}