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/MacAppearance.swift

@@ -0,0 +1,75 @@
+// MacAppearance.swift
+// OSGKeyboard · Mac
+//
+// User-facing light / dark preference for the desktop app. Stored locally
+// (Mac-only switch for now); can be promoted into `SyncedAppSettings` later
+// if iPad / cross-device appearance sync is wanted.
+
+import AppKit
+import SwiftUI
+
+/// How the macOS app resolves its colour scheme.
+enum MacAppearancePreference: String, CaseIterable, Identifiable {
+    case system
+    case light
+    case dark
+
+    var id: String { rawValue }
+
+    /// `nil` means "follow the system", matching SwiftUI's convention where
+    /// `preferredColorScheme(nil)` defers to the environment.
+    var colorScheme: ColorScheme? {
+        switch self {
+        case .system: return nil
+        case .light:  return .light
+        case .dark:   return .dark
+        }
+    }
+
+    /// The matching AppKit appearance so window chrome, traffic lights and
+    /// the menu-bar popover follow the same choice as the SwiftUI content.
+    var nsAppearance: NSAppearance? {
+        switch self {
+        case .system: return nil
+        case .light:  return NSAppearance(named: .aqua)
+        case .dark:   return NSAppearance(named: .darkAqua)
+        }
+    }
+
+    var labelKey: String {
+        switch self {
+        case .system: return "mac.appearance.system"
+        case .light:  return "mac.appearance.light"
+        case .dark:   return "mac.appearance.dark"
+        }
+    }
+
+    var systemImage: String {
+        switch self {
+        case .system: return "circle.lefthalf.filled"
+        case .light:  return "sun.max"
+        case .dark:   return "moon"
+        }
+    }
+
+    /// `@AppStorage` key shared by the app shell and the settings switch.
+    static let storageKey = "mac.appearancePreference"
+
+    static var current: MacAppearancePreference {
+        MacAppearancePreference(
+            rawValue: UserDefaults.standard.string(forKey: storageKey) ?? ""
+        ) ?? .system
+    }
+
+    /// Push the preference into AppKit so non-SwiftUI chrome (title bar,
+    /// popover) tracks it too. Safe to call on the main actor at any time.
+    @MainActor
+    static func applyToApp(_ preference: MacAppearancePreference) {
+        NSApp?.appearance = preference.nsAppearance
+        NSApp?.windows.forEach { window in
+            window.appearance = preference.nsAppearance
+            window.contentView?.needsDisplay = true
+            window.contentView?.subviews.forEach { $0.needsDisplay = true }
+        }
+    }
+}