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)!