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/Models/ProviderConfig.swift

@@ -36,7 +36,11 @@ public final class ProviderConfig: ObservableObject, @unchecked Sendable {
         didSet {
             guard oldValue != apiKey, !isSyncingProviderAPIKey else { return }
             do {
-                try Keychain.setAPIKey(apiKey, for: providerId)
+                try Keychain.setAPIKey(
+                    apiKey,
+                    for: providerId,
+                    useICloudSync: configuration.settingsICloudSyncEnabled
+                )
             } catch {
                 OSGLog.config.warning("Keychain write failed: \(error.localizedDescription, privacy: .public)")
             }