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.

OSGKeyboardMac/MacL10n.swift

@@ -0,0 +1,17 @@
+// MacL10n.swift
+// OSGKeyboard · Mac
+//
+// Bilingual UI strings for the macOS app. Reuses Shared.strings and
+// respects the same `AppUILanguage` override as the iOS settings page.
+
+import Foundation
+
+enum MacL10n {
+    static func string(_ key: String, language: AppUILanguage? = nil) -> String {
+        SharedL10n.string(key, language: language)
+    }
+
+    static func format(_ key: String, language: AppUILanguage? = nil, _ args: CVarArg...) -> String {
+        SharedL10n.format(key, language: language, args)
+    }
+}