fix(main): force hard exit on app close to prevent zombie processes

[hennring]

Problem

When closing the application window, the Renderer process terminates, but the remaining Electron/Chromium background tasks (Main, GPU, and Network processes) stay alive in the system's Task Manager.

Because of the app's single-instance lock implementation, these lingering "zombie" processes silently block users from reopening the application unless they manually kill the tasks via Task Manager. Furthermore, this resource lock causes file-permission errors (EPERM) during local build routines (npm run build:desktop).

The root cause is that hardware-heavy operations (like Spout/NDI) or active plugin child processes occasionally block Electron's graceful app.quit() lifecycle from successfully completing the event loop.

Solution

This PR refactors the cleanupAndQuit() function in the main process to ensure a guaranteed, unblockable application shutdown:

1. Replaced app.quit() with app.exit(0): Once the cleanup logic has run, this forces all 3 active Chromium/Node tasks to exit immediately, bypassing Electron's easily obstructed graceful closing routine.
2. Robust Exception Handling: Wrapped the NDI and plugin iteration blocks inside isolated try/catch statements. If a native addon throws an unhandled error or if plugins is undefined/null, the sequence will safely continue instead of crashing halfway through.
3. Forced Plugin Destruction: Changed the plugin termination signal to SIGKILL to forcefully drop any active hardware or file handles.
4. Streamlined Execution: Removed the deferred setTimeout block to maintain a clean, synchronous, and predictable linear execution flow.

Linked Issues

Fixes #3

Testing Checklist

• Verified that the app UI closes successfully.
• Confirmed via Task Manager that all 3 background processes (Main, GPU, Network) disappear instantly upon closing.
• Verified that the application can be reopened immediately after closing.
• Ran npm run build:desktop without encountering EPERM file-locking issues.