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/AccountUserDefaults.swift

@@ -0,0 +1,66 @@
+//
+// 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
+
+/// A store that scopes all of its values to a single account.
+///
+/// Each value keeps its own key in the parent store, prefixed with the user ID so that writes
+/// remain atomic across the app and its extensions.
+final nonisolated class AccountUserDefaults: UserDefaultsProtocol, @unchecked Sendable {
+    private let keyPrefix: String
+    private let store: UserDefaultsProtocol
+    
+    init(userID: String, store: UserDefaultsProtocol) {
+        keyPrefix = "account[\(userID)]."
+        self.store = store
+    }
+    
+    // MARK: Storage
+    
+    func data(forKey key: String) -> Data? {
+        store.data(forKey: keyPrefix + key)
+    }
+    
+    func object(forKey key: String) -> Any? {
+        store.object(forKey: keyPrefix + key)
+    }
+    
+    func removeObject(forKey key: String) {
+        store.removeObject(forKey: keyPrefix + key)
+    }
+    
+    func set(_ value: Any?, forKey key: String) {
+        store.set(value, forKey: keyPrefix + key)
+    }
+    
+    // MARK: - Management
+    
+    func migrateAppSettingsValue(forKey key: String) {
+        guard let value = store.object(forKey: key) else {
+            MXLog.info("Ignoring \(key), not set.")
+            return
+        }
+        
+        store.set(value, forKey: keyPrefix + key)
+        store.removeObject(forKey: key)
+    }
+    
+    func reset() {
+        for key in dictionaryRepresentation().keys {
+            removeObject(forKey: key)
+        }
+    }
+    
+    func dictionaryRepresentation() -> [String: Any] {
+        var representation = [String: Any]()
+        for (key, value) in store.dictionaryRepresentation() where key.hasPrefix(keyPrefix) {
+            representation[String(key.dropFirst(keyPrefix.count))] = value
+        }
+        return representation
+    }
+}