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/VolatileUserDefaults.swift

@@ -0,0 +1,49 @@
+//
+// 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 VolatileUserDefaults: UserDefaultsProtocol, Sendable {
+    private let lock = NSLock()
+    
+    private nonisolated(unsafe) var _storage: [String: Any]
+    private(set) var storage: [String: Any] {
+        get { lock.withLock { _storage } }
+        set { lock.withLock { _storage = newValue } }
+    }
+    
+    init(initialValues: [String: Any] = [:]) {
+        lock.lock()
+        defer { lock.unlock() }
+        
+        _storage = initialValues
+    }
+    
+    func data(forKey key: String) -> Data? {
+        storage[key] as? Data
+    }
+    
+    func object(forKey key: String) -> Any? {
+        storage[key]
+    }
+    
+    func removeObject(forKey key: String) {
+        storage.removeValue(forKey: key)
+    }
+    
+    func set(_ value: Any?, forKey key: String) {
+        storage[key] = value
+    }
+    
+    func reset() {
+        storage = [:]
+    }
+    
+    func dictionaryRepresentation() -> [String: Any] {
+        storage
+    }
+}