feat(macos): local ASR model manager, menu-bar polish, and release 0.5.2

Adds a bundled local ASR model catalog for the macOS app with one-click
Sherpa Qwen3 / SenseVoice downloads (pause/resume, inline actions) and a
shared model storage directory used by MLX Qwen3. Fixes the light-mode
sidebar material and makes the menu-bar icon follow the system appearance
with a refreshed status mark. Renames the built product to OSGKeyboard.app.

Bumps version to 0.5.2 (build 19).

OSGKeyboardShared/Services/FlowSessionDarwin.swift

@@ -8,6 +8,8 @@ import Foundation
 
 public enum FlowSessionDarwin {
     public static let notificationName = "com.osgkeyboard.flow.session.changed"
+    /// Posted when the keyboard writes a command for the host app.
+    public static let commandNotificationName = "com.osgkeyboard.flow.command.changed"
     /// Posted when the host app writes a transcription result or error.
     public static let transcriptionNotificationName = "com.osgkeyboard.flow.transcription.changed"
     /// Posted when the host app publishes or clears the ready contract.
@@ -23,6 +25,16 @@ public enum FlowSessionDarwin {
         )
     }
 
+    public static func postCommandChanged() {
+        CFNotificationCenterPostNotification(
+            CFNotificationCenterGetDarwinNotifyCenter(),
+            CFNotificationName(commandNotificationName as CFString),
+            nil,
+            nil,
+            true
+        )
+    }
+
     public static func postTranscriptionChanged() {
         CFNotificationCenterPostNotification(
             CFNotificationCenterGetDarwinNotifyCenter(),