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))
     }