feat(gui): add Windows GUI application with Tauri

Built ostp-gui.exe - graphical VPN client with buttons!

Features:
- Connect/Disconnect button
- Server selection dropdown
- Real-time stats (Upload/Download/Ping)
- Settings panel
- Beautiful interface (Tauri 2.0 + HTML/CSS/JS)
- IPC with ostp-daemon via Named Pipe

UI Components:
- Main window: 450x600 (not resizable)
- Status indicator (Connected/Disconnected)
- Server selection list
- Stats grid display
- Settings button

Files:
- ostp-gui.exe: 5.79 MB (NEW!)
- ostp-client.exe: 1.64 MB (CLI)
- ostp-daemon.exe: 0.53 MB (service)
- ostp-installer.exe: 0.37 MB

Changes:
1. Fixed tauri.conf.json paths (frontendDist: ./ui)
2. Removed protocol-asset feature from Cargo.toml
3. Added icons/icon.ico (user provided)
4. Built successfully with Tauri 2.0
5. Updated SHA256SUMS.txt with GUI checksum
6. Recreated ostp-client-windows-x64.zip (3.64 MB)

Usage:
- Just run ostp-gui.exe - NO command line needed!
- Select server, click Connect - thats it!

Location: dist/windows-x64/ostp-gui.exe
SHA256: 5880cfeeb7fd2dcf1b033b7e2514a56eb45b3220d5c2c783f5a7f2295bd28ca4

dist/windows-x64/SHA256SUMS.txt

@@ -1,3 +1,4 @@
+5880cfeeb7fd2dcf1b033b7e2514a56eb45b3220d5c2c783f5a7f2295bd28ca4  ostp-gui.exe
 5612ae4503467d6d7b51c067f533706bc65b7b7ac41fccc3efaf3eda2576b92a  ostp-client.exe
 2e5c4dda1b0e9d1807892a3de587e4875906bdbe5343d5b83b5f33488d6e6696  ostp-daemon.exe
 8d0c759d4466e8b189ef161d6e0caac823e690bf5a82ee48308975d163728d7f  ostp-installer.exe