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/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)
+        }
     }
 }