refactor(concurrency): isolate ProviderConfig to the main actor and drop unneeded @unchecked Sendable

- ProviderConfig is now @MainActor instead of @unchecked Sendable; AppL10n reads the persisted UI language key directly so off-main string lookups stay valid.
- EnglishLearningStore serializes its read-modify-write with a type-wide lock (several instances share one defaults key).
- URLSession-only transports, GatewayGrantKeychainStore and Keychain conform to Sendable without @unchecked. UserDefaults/DeviceCheck wrappers keep @unchecked because those SDK types are not Sendable.

OSGKeyboard/Services/PublicContentHTTPTransport.swift

@@ -12,7 +12,7 @@ protocol PublicContentHTTPTransport: Sendable {
 
 final class URLSessionPublicContentHTTPTransport:
     PublicContentHTTPTransport,
-    @unchecked Sendable {
+    Sendable {
     private let session: URLSession
 
     init(session: URLSession = .shared) {

OSGKeyboard/Utilities/AppL10n.swift

@@ -30,9 +30,16 @@ enum AppL10n {
     /// screen that exists so a provisioning mistake does *not* become a crash
     /// loop. `.auto` follows the system language, which is the right answer
     /// when no stored preference is reachable anyway.
+    ///
+    /// Reads the persisted key directly rather than `ProviderConfig.shared`
+    /// (main-actor isolated) because localized strings are also resolved off
+    /// the main actor. `ProviderConfig` writes this key synchronously in its
+    /// `uiLanguage` didSet, so both paths observe the same value.
     private static var resolvedUILanguage: AppUILanguage {
-        guard AppGroup.isAvailable else { return .auto }
+        guard let defaults = AppGroup.defaultsIfAvailable else { return .auto }
-        return ProviderConfig.shared.uiLanguage
+        return AppUILanguage.fromStored(
+            defaults.string(forKey: AppGroupConfiguration.Keys.uiLanguage)
+        )
     }
 
     static func format(

OSGKeyboard/Views/SettingsSecondaryPages.swift

@@ -91,6 +91,7 @@ struct SettingsVersionRow: View {
 
 // MARK: - Config entry summaries (shown on Settings home)
 
+@MainActor
 enum SettingsConfigSummary {
     static func speechRecognition(config: ProviderConfig) -> String {
         if config.engineMode == "local" {

OSGKeyboardHostSupport/Features/Account/AccountAPIClient.swift

@@ -13,7 +13,7 @@ public protocol AccountHTTPTransport: Sendable {
     func data(for request: URLRequest) async throws -> (Data, HTTPURLResponse)
 }
 
-public final class URLSessionAccountHTTPTransport: AccountHTTPTransport, @unchecked Sendable {
+public final class URLSessionAccountHTTPTransport: AccountHTTPTransport, Sendable {
     private let session: URLSession
 
     public init(session: URLSession = .shared) {

OSGKeyboardShared/Core/Configuration/LiveConfigurationStore.swift

@@ -97,6 +97,7 @@ public struct LiveConfigurationSnapshot: @unchecked Sendable {
     }
 
     /// Build from live `ProviderConfig` plus persisted App Group extras.
+    @MainActor
     public init(config: ProviderConfig, fallback: AppGroupStore) {
         self.init(
             providerId: config.providerId,
@@ -136,6 +137,7 @@ public struct LiveConfigurationStore: ConfigurationStore, @unchecked Sendable {
         self.init(snapshot: LiveConfigurationSnapshot(store: store))
     }
 
+    @MainActor
     public init(config: ProviderConfig, fallback: AppGroupStore) {
         self.init(snapshot: LiveConfigurationSnapshot(config: config, fallback: fallback))
     }

OSGKeyboardShared/Features/Analytics/AnalyticsUploadCoordinator.swift

@@ -6,7 +6,7 @@
 
 import Foundation
 
-public final class URLSessionAnalyticsNetwork: AnalyticsNetworking, @unchecked Sendable {
+public final class URLSessionAnalyticsNetwork: AnalyticsNetworking, Sendable {
     private let session: URLSession
 
     public init(session: URLSession = .shared) {

OSGKeyboardShared/Features/ManagedGateway/GatewayGrantCredentialStore.swift

@@ -13,7 +13,7 @@ public protocol GatewayGrantCredentialStore: Sendable {
     func delete() async throws
 }
 
-public struct GatewayGrantKeychainStore: GatewayGrantCredentialStore, @unchecked Sendable {
+public struct GatewayGrantKeychainStore: GatewayGrantCredentialStore, Sendable {
     public enum Slot: String, Sendable {
         case account = "scope-limited.active"
         case oobe = "scope-limited.oobe"

OSGKeyboardShared/Models/ProviderConfig.swift

@@ -12,10 +12,10 @@
 import Combine
 import Foundation
 
-/// UI-owned ObservableObject; construct and mutate it on the main thread.
+/// UI-owned ObservableObject, isolated to the main actor. Credential observers
-/// `@unchecked Sendable` does not make `@Published` thread-safe. Credential
+/// write only Keychain, while non-secret configuration uses App Group defaults.
-/// observers write only Keychain, while non-secret configuration uses App Group defaults.
+@MainActor
-public final class ProviderConfig: ObservableObject, @unchecked Sendable {
+public final class ProviderConfig: ObservableObject {
     public static let shared = ProviderConfig()
 
     @Published public var providerId: String {

OSGKeyboardShared/Services/Keychain.swift

@@ -12,7 +12,7 @@
 import Foundation
 import Security
 
-public enum Keychain: @unchecked Sendable {
+public enum Keychain: Sendable {
 
     public enum KeychainError: Error, Sendable, Equatable {
         case unexpectedStatus(OSStatus)

OSGKeyboardShared/Services/LLMClient.swift

@@ -418,6 +418,7 @@ public struct OpenAICompatibleClient: LLMClient {
 
 public enum LLMClientFactory {
     /// Build a client from the current `ProviderConfig`.
+    @MainActor
     public static func make(from config: ProviderConfig) -> LLMClient {
         make(
             providerId: config.providerId,

OSGKeyboardShared/Typing/EnglishLearningStore.swift

@@ -10,6 +10,10 @@ import Foundation
 public final class EnglishLearningStore: @unchecked Sendable {
     public static let defaultsKey = "englishTyping.learnedBoosts.v1"
 
+    /// Static because several instances share the same defaults key; a
+    /// per-instance lock would not serialize their read-modify-write.
+    private static let mutationLock = NSLock()
+
     private let defaults: UserDefaults
 
     public init(defaults: UserDefaults = AppGroupStore().defaults) {
@@ -42,8 +46,10 @@ public final class EnglishLearningStore: @unchecked Sendable {
     private func mutate(word: String, delta: Int) {
         let key = word.lowercased()
         guard !key.isEmpty else { return }
+        Self.mutationLock.withLock {
             var map = snapshot()
             map[key] = min(10_000, (map[key] ?? 0) + delta)
             defaults.set(map, forKey: Self.defaultsKey)
         }
+    }
 }

OSGKeyboardTests/KeychainTests.swift

@@ -235,6 +235,7 @@ final class KeychainTests: XCTestCase {
     /// value must be moved to the Keychain and removed from UserDefaults
     /// — otherwise a fresh install in a clean simulator would inherit the
     /// stale plaintext value.
+    @MainActor
     func testLegacyUserDefaultsKeyIsMigratedToKeychain() {
         let suiteName = "group.com.osgkeyboard.shared.tests.migration.\(UUID().uuidString)"
         let defaults = UserDefaults(suiteName: suiteName)!
@@ -271,6 +272,7 @@ final class KeychainTests: XCTestCase {
     /// — possible on a downgrade or a torn update — Keychain wins. We
     /// don't delete the UserDefaults value, but the running app reads from
     /// the Keychain only.
+    @MainActor
     func testKeychainWinsOverLegacyWhenBothPresent() {
         let suiteName = "group.com.osgkeyboard.shared.tests.migration2.\(UUID().uuidString)"
         let defaults = UserDefaults(suiteName: suiteName)!

OSGKeyboardTests/LLMClientTests.swift

@@ -45,6 +45,7 @@ final class LLMClientTests: XCTestCase {
 
     // MARK: - ProviderConfig persistence
 
+    @MainActor
     func testProviderConfigPersistsAcrossInstances() {
         let suiteName = "group.com.osgkeyboard.shared.tests.persist.\(UUID().uuidString)"
         let defaults = UserDefaults(suiteName: suiteName)!
@@ -64,6 +65,7 @@ final class LLMClientTests: XCTestCase {
     }
 
     /// Local ASR needs no cloud ASR key, but polish still requires a user API key.
+    @MainActor
     func testLocalEngineWithoutAPIKeyIsNotPolishConfigured() {
         let suiteName = "group.com.osgkeyboard.shared.tests.isconfigured.\(UUID().uuidString)"
         let defaults = UserDefaults(suiteName: suiteName)!
@@ -299,6 +301,7 @@ final class LLMClientTests: XCTestCase {
 
     /// Cross-process App Group contract: what `ProviderConfig` writes must
     /// be readable through `AppGroupStore` on the same suite.
+    @MainActor
     func testAppGroupCrossProcessLegacyOffModeMigratesToPolish() async {
         let suiteName = "group.com.osgkeyboard.shared.tests.\(UUID().uuidString)"
         let defaults = UserDefaults(suiteName: suiteName)!
@@ -377,6 +380,7 @@ final class LLMClientTests: XCTestCase {
         )
     }
 
+    @MainActor
     func testLiveConfigurationStoreSnapshotsAnyConfigurationStore() throws {
         let suiteName = "group.com.osgkeyboard.shared.tests.\(UUID().uuidString)"
         let defaults = UserDefaults(suiteName: suiteName)!
@@ -561,6 +565,7 @@ final class LLMClientTests: XCTestCase {
         XCTAssertNil(body["thinking_config"])
     }
 
+    @MainActor
     func testLLMThinkingDefaultIsOffInFreshConfiguration() {
         let suiteName = "group.com.osgkeyboard.shared.tests.thinking.\(UUID().uuidString)"
         let defaults = UserDefaults(suiteName: suiteName)!