fix: kill orphaned node.exe before NSIS install to prevent file lock error

[web3dev1337]

Summary

• Fixes the Windows installer error: "Error opening file for writing: C:\Users[User]\AppData\Local\agent-workspace\resources\backend\node\node.exe"
• Root cause: the bundled node.exe (spawned as the backend server) remains running as an orphaned process after app close or uninstall, holding a file lock on itself — the NSIS installer cannot overwrite it
• Adds a custom NSIS preinstall hook (src-tauri/windows/hooks.nsh) that taskkill /F both Agent Workspace.exe and node.exe before file extraction begins
• Same cleanup added to the preuninstall hook for clean removal

Files changed

• src-tauri/windows/hooks.nsh — new NSIS hook file with NSIS_HOOK_PREINSTALL and NSIS_HOOK_PREUNINSTALL macros
• src-tauri/tauri.conf.json — added bundle.windows.nsis.installerHooks pointing to the hook file

Test plan

• Build NSIS installer (npm run tauri:build or npm run tauri:build:fast)
• Install the app, start it (so node.exe is running)
• Close the app but leave node.exe orphaned (or don't close — either way)
• Run the installer again — should succeed without "Error opening file for writing"
• Verify the app launches correctly after reinstall

Note

The taskkill /IM node.exe /F is broad and will kill all node.exe processes on the machine. This is acceptable because:

1. It only runs during install/uninstall (not at runtime)
2. Users expect installers to briefly interrupt other processes
3. The alternative (users manually killing node via Task Manager) is the current workaround

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