chore: ipc cleanup

[jkleinsc]

This PR reduces the scope of IPC exposed to the renderer process and adds input validation for filesystem-related handlers.

There were events inipcMainEvents that silently accepted fire-and-forget messages from the renderer even though no handler consumed them, so they have been removed.

Sender validation: The IpcMainManager constructor now verifies that every inbound ipcMain.on event originates from a window tracked by BrowserWindow.fromWebContents. Messages from WebViews, sub-frames, or detached/untracked windows are silently dropped.

Added input validation to three filesystem handlers before passing renderer-supplied values to fs operations.

CLEANUP_DIRECTORY — rejects any path that does not resolve inside os.tmpdir(), preventing path traversal to arbitrary filesystem locations.

DELETE_USER_DATA — rejects names containing path separators (/, path.sep) or .., so the renderer cannot escape app.getPath('appData').

PATH_EXISTS — type-checks the argument before calling fs.existsSync.

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[coveralls]

coverage: 88.606% (-0.4%) from 89.016% — ipc-cleanup into main

[coveralls]

coverage: 88.986% (-0.4%) from 89.358% — ipc-cleanup into main