Merge worktree-windows-icon-tray-fix: Windows 桌面/托盘/任务栏图标 + 关窗不断连接

- 关窗(X/Alt+F4)隐藏到托盘并保持隧道,真退出只走托盘菜单(Platform.isWindows 隔离)
- 窗口标题设「穿山甲」(无 VPN 字眼)
- Windows 托盘改用 .ico(Win32 只认 ico,PNG 空白),mac/Linux 仍 PNG
- 更新 app_icon.ico / tray_icon.ico / Runner.rc / installer

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/pubspec.yaml

@@ -1,7 +1,7 @@
 name: pangolin_vpn
 description: 穿山甲 · Pangolin — 极简、稳定、跨平台网络加速客户端。
 publish_to: "none"
-version: 1.0.0+1
+version: 1.0.2+3
 
 environment:
   sdk: ^3.5.0
@@ -55,6 +55,7 @@ flutter:
     - assets/logo-wordmark.svg
     - assets/app-icon.svg
     - assets/tray_icon.png
+    - assets/tray_icon.ico   # Windows 托盘专用(Win32 只认 .ico)
 
   # ── 字体(本地打包,全平台离线)──
   # 拉丁全量(Sora/Manrope/JetBrains Mono)+ Noto Sans SC 子集(GB2312 6763 字,单 Regular