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.

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.
-/// `@unchecked Sendable` does not make `@Published` thread-safe. Credential
-/// observers write only Keychain, while non-secret configuration uses App Group defaults.
-public final class ProviderConfig: ObservableObject, @unchecked Sendable {
+/// UI-owned ObservableObject, isolated to the main actor. Credential observers
+/// write only Keychain, while non-secret configuration uses App Group defaults.
+@MainActor
+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 }
-        var map = snapshot()
-        map[key] = min(10_000, (map[key] ?? 0) + delta)
-        defaults.set(map, forKey: Self.defaultsKey)
+        Self.mutationLock.withLock {
+            var map = snapshot()
+            map[key] = min(10_000, (map[key] ?? 0) + delta)
+            defaults.set(map, forKey: Self.defaultsKey)
+        }
     }
 }