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 }
-        return ProviderConfig.shared.uiLanguage
+        guard let defaults = AppGroup.defaultsIfAvailable else { return .auto }
+        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" {