feat(panels): support grouped user menu items with separate dropdown …

[tomjamon]

Description

Panels currently only support a single flat userMenuItems() array. Items below the theme switcher all appear in one <x-filament::dropdown.list>, which makes dense menus (e.g. account + locales + support) hard to scan.

This PR adds an optional grouped registration shape: pass a list of arrays, where each inner array is one registration group. When more than one group exists, the panel renders multiple <x-filament::dropdown.list> blocks after the theme switcher (same hooks and item rendering as today).

Backward compatibility

• A single flat userMenuItems([...]) call behaves exactly as before.
• Multiple flat userMenuItems([...]) calls still merge new items into the last group (existing pattern preserved).

Logout

If no group defines logout, the default logout action is appended to the last after-theme group so users always keep a sign-out entry.

Implementation note

getUserMenuItems() is memoized per request on the panel so profile/logout and other resolved actions are the same instances whether the menu is built as a flat list or split into groups. That keeps Livewire’s defaultView(Action::GROUPED_VIEW) and cacheAction() aligned with what Blade renders (avoids a second getUserLogoutMenuItem() instance rendering as a primary button).

Visual changes

Before: One continuous list under the theme switcher for all sort() >= 0 items.

After (when using grouped registration): Visually separated lists—one per registered group—still using the same dropdown list item styling.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.