feat(desktop): add native Tauri desktop app and root Berry.bat launcher

[BerryUIKI]

Summary

• Introduces native Windows desktop application shell powered by Tauri 2 and Windows WebView2 in \rontend/src-tauri.
• Embedded backend supervisor in Tauri (\src-tauri/src/backend.rs) automatically launches Berry core, verifies readiness via /health, and manages graceful shutdown without spawning command console windows.
• In desktop mode, Berry AI Studio renders directly inside a native desktop window (1440x900 resizable, centered, min 1024x700), eliminating the need to open web browser tabs.
• Added top-level \Berry.bat\ at repository root for immediate one-click desktop launch.
• Updated release packaging script \scripts/package-windows-release.ps1\ to bundle \Berry AI Studio.exe\ and update portable distribution.