feat(macos): add macOS menu-bar app and harden cross-device iCloud sync

Introduce a standalone macOS menu-bar app (OSGKeyboardMac) that reuses the
platform-agnostic OSGKeyboardShared core: record -> cloud/local ASR -> polish
-> insert. Local mode uses Qwen3-ASR via mlx-swift-asr (macOS 15+, Apple
Silicon); iOS targets stay zero-SPM.

Harden iCloud sync for multi-device correctness:
- Per-field settings merge (appSettings.v2) so concurrent edits no longer
  clobber each other's unrelated fields.
- Per-device usage statistics (G-Counter) that sum instead of max().
- Tombstoned dictionary/history merge so deletes propagate and entries can't
  resurrect.
- API keys replicate via iCloud Keychain, never iCloud KVS JSON; pulling a
  legacy blob without key fields no longer wipes local Keychain entries.
- Add a low-risk "Sync Now" action in Settings.

Fix Flow keyboard mic state: stay orange until the host publishes a real ready
contract, share a single MicVoiceAvailability gate, and self-heal stale
cross-process heartbeat jitter instead of getting stuck.

Extract shared storage (SpeechHistoryStore/UsageStatisticsStore,
ConfigurationStore) into OSGKeyboardShared and add tests for the new
sync/merge logic.

OSGKeyboardShared/Services/PolishingService.swift

@@ -48,7 +48,7 @@ public actor PolishingService {
         case translate(targetLocaleId: String)
     }
 
-    private let store: AppGroupStore
+    private let store: any ConfigurationStore
     private let timeout: TimeInterval
     /// Optional injected client (mostly for testing). When nil we build
     /// one from `store.makeClient()` per call.
@@ -60,7 +60,7 @@ public actor PolishingService {
     /// own slack on top of the length-scaled budget in `polishRemote`, so
     /// no `+1` is baked in here.
     public init(
-        store: AppGroupStore = AppGroupStore(),
+        store: any ConfigurationStore = AppGroupStore(),
         client: LLMClient? = nil,
         timeout: TimeInterval? = nil
     ) {
@@ -346,7 +346,7 @@ public actor PolishingService {
     }
 
     internal static func resolvedProviderId(
-        store: AppGroupStore,
+        store: any ConfigurationStore,
         providerIdOverride: String?
     ) -> String {
         if let providerIdOverride {
@@ -360,7 +360,7 @@ public actor PolishingService {
     }
 
     internal static func resolveLLMEndpoint(
-        store: AppGroupStore,
+        store: any ConfigurationStore,
         preset: LLMProvider,
         providerIdOverride: String?
     ) -> (baseURL: String, model: String) {