[6.2] [AI] Backend: Save column visibility preferences per user account in list views

[MacJoom]

Adds plg_system_usercolumns (system plugin) that stores each logged-in user's hidden-column preferences in #__users.params (key: hidden_tablecolumns) - only active in the backend. The table-columns.es6.js saveState() now POSTs to com_ajax on every toggle (fire-and-forget), and loadState() reads server-injected state first so preferences follow the user across browsers and sessions, falling back to localStorage for guests and anonymous users.

Pull Request resolves # .

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

• build/media_source/system/js/table-columns.es6.js
  • saveState() now additionally POSTs the hidden-column list to the server
    via com_ajax (fire-and-forget, no UX impact)
  • loadState() checks server-injected state first (key: table.columns.state);
    falls back to localStorage for guests
• plugins/system/usercolumns/ — new system plugin plg_system_usercolumns
  • onBeforeRender: reads tablecolumns from #__users.params and injects it
    into the page via Document::addScriptOptions() — no extra request needed
  • onAjaxUsercolumns: handles the AJAX POST from the JS, validates the CSRF
    token, sanitises input, and saves preferences to #__users.params

Testing Instructions

1. Install the plugin via System → Discover (click Discover, then install
   System – User Column Preferences)
2. Enable the plugin in System → Plugins - User Column Preferences
3. Go to any admin list view (e.g. Articles, Modules, Users)
4. Use the Columns button to hide one or more columns — wait a moment for the
   AJAX call to complete
5. Use the "Save" button to write the changes to the DB
6. Open the same list view in a different browser or private/incognito tab and
   log in
7. Verify the same columns are hidden

Actual result BEFORE applying this Pull Request

Column visibility is stored in localStorage only. Preferences are lost when
switching browsers, clearing cache, switching devices, or using a private tab.

Expected result AFTER applying this Pull Request

Expected result AFTER applying this Pull Request

Column visibility is persisted in the user account. Preferences are consistent
across all browsers and sessions for the same user account.

Link to documentations

Please select:

• Documentation link for guide.joomla.org:

• No documentation changes for guide.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed