Fresh install lacks plugins directory, causing plugin installation from GUI to fail

[CelestialZ-75]

Title: Fresh install lacks plugins directory, causing plugin installation from GUI to fail

name: Bug report
about: Create a report to help us improve EEGLAB
title: 'Missing plugins directory after fresh install prevents GUI plugin installation'
labels: 'bug'
assignees: 'zhou'

Description

After a fresh EEGLAB installation (e.g., via MATLAB Add-Ons Manager or plugin_askinstall/plugin_install), the root EEGLAB directory may not contain a plugins/ folder. When users attempt to install plugins via the GUI menu (File > Manage EEGLAB extensions), the installation fails because the target directory does not exist.

The plugin_askinstall and plugin_install functions should create the plugins/ directory if it is missing, rather than assuming it already exists.

#### Steps to Reproduce

1. Fresh install EEGLAB (e.g., eeglab nogui from a freshly downloaded copy).
2. From the GUI, go to File > Manage EEGLAB extensions.
3. Select any plugin and attempt to install.

#### Expected behavior: Plugin installs successfully; the `plugins/` directory is created automatically if absent.
#### Actual behavior: The installation fails with an error because the `plugins/` target directory does not exist.
#### Versions

OS version	Windows / macOS / Linux
Matlab version	any
EEGLAB version	any recent (2024+)