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.

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)!