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/MacTheme.swift

@@ -0,0 +1,49 @@
+// MacTheme.swift
+// OSGKeyboard · Mac
+//
+// System-native colour palette for the desktop app. Instead of the custom
+// near-black brand palette, the Mac app maps every design token onto AppKit
+// semantic colours (`Color(nsColor:)`), which adapt to light / dark on their
+// own. The brand green is kept only as the accent. This gives the app the
+// same zero-colour-difference, System-Settings / Notes look on both
+// appearances while reusing every existing `palette.X` call site.
+
+import AppKit
+import SwiftUI
+
+enum MacSystemPalette {
+    /// A `ThemePalette` whose surfaces and text resolve to AppKit semantic
+    /// colours. Because those colours are dynamic, a single value renders
+    /// correctly under both light and dark (driven by `preferredColorScheme`).
+    static let palette = ThemePalette(
+        background:      Color(nsColor: .windowBackgroundColor),
+        surface:         Color(nsColor: .controlBackgroundColor),
+        surfaceElevated: Color(nsColor: .unemphasizedSelectedContentBackgroundColor),
+        surfaceMuted:    Color(nsColor: .underPageBackgroundColor),
+
+        accent:          Palette.accent,
+        accentMuted:     Palette.accent.opacity(0.16),
+        accentGlow:      Palette.accent.opacity(0.35),
+
+        danger:          Color(nsColor: .systemRed),
+        success:         Palette.accent,
+        warning:         Color(nsColor: .systemOrange),
+
+        textPrimary:     Color(nsColor: .labelColor),
+        textSecondary:   Color(nsColor: .secondaryLabelColor),
+        textTertiary:    Color(nsColor: .tertiaryLabelColor),
+        textOnAccent:    Color.white,
+
+        divider:         Color(nsColor: .separatorColor),
+        dividerStrong:   Color(nsColor: .separatorColor),
+
+        recordRed:       Color(nsColor: .systemRed)
+    )
+}
+
+extension View {
+    /// Injects the system-native palette used across the macOS app.
+    func macSystemPalette() -> some View {
+        environment(\.themePalette, MacSystemPalette.palette)
+    }
+}