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/AccountSettings.swift

@@ -0,0 +1,42 @@
+//
+// 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 Combine
+import Foundation
+import Macros
+
+final nonisolated class AccountSettings: @unchecked Sendable {
+    /// UserDefaults to be used on reads and writes.
+    private let store: AccountUserDefaults
+    private let userID: String
+    
+    // MARK: Session
+    
+    @UserPreference(defaultValue: false)
+    var hasRunIdentityConfirmationOnboarding: Bool
+    
+    // MARK: Search
+    
+    /// The queries the user searched for and the rooms they opened from the results, most recent first.
+    @UserPreference(defaultValue: [SearchBreadcrumb]())
+    var searchBreadcrumbs: [SearchBreadcrumb]
+    
+    init(userID: String, store: UserDefaultsProtocol) {
+        self.store = .init(userID: userID, store: store)
+        self.userID = userID
+    }
+    
+    func migrateAppSettingsValue(_ keyPath: KeyPath<AccountSettings, UserPreferenceKey>) {
+        let storeKey = self[keyPath: keyPath].rawValue
+        store.migrateAppSettingsValue(forKey: storeKey)
+    }
+    
+    func reset() {
+        MXLog.warning("Resetting AccountSettings for \(userID).")
+        store.reset()
+    }
+}