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/KeyboardState.swift

@@ -82,7 +82,10 @@ public final class KeyboardState: ObservableObject {
     @Published public var utteranceRemainingSeconds: Int = Int(FlowSessionKeys.maxUtteranceDuration)
     /// Whether the host app's Flow voice session is live and reachable (fresh
     /// heartbeat). Do not use the App Group session flag alone for UI gating.
+    /// Prefer `micVoiceAvailability` for mic color and tap behavior.
     @Published public var flowSessionActive: Bool = false
+    /// Unified mic color / tap / hint source for the keyboard extension.
+    @Published public var micVoiceAvailability: MicVoiceAvailability = .unavailable(.hostNotReady)
     /// When true, the mic is intentionally disabled (e.g. cloud engine
     /// selected but the provider-specific API key is missing).
     @Published public var micDisabled: Bool = false