fix(desktop): add single-instance plugin to prevent multiple windows

[Junseo5]

Problem

When launching the desktop app on Windows with plugins that take time to load (like oh-my-opencode), multiple instances spawn continuously because there's no single-instance guard. This causes 10+ windows to open.

Solution

Add tauri-plugin-single-instance which:

• Prevents multiple app instances from running simultaneously
• Focuses existing window when user tries to launch another instance
• Unminimizes window if it was minimized

Changes

• packages/desktop/src-tauri/Cargo.toml: Add tauri-plugin-single-instance = "2"
• packages/desktop/src-tauri/src/lib.rs: Initialize single-instance plugin with focus handler

Testing

1. Build desktop app with slow-loading plugins
2. Try launching app multiple times quickly
3. Expected: Only one window opens, subsequent launches focus existing window

Fixes #6965