Add Closure support to sidebarWidth() and collapsedSidebarWidth()

[ashraful19]

Description

Makes sidebarWidth() and collapsedSidebarWidth() accept a Closure for dynamic evaluation (e.g., per-tenant/user sidebar sizing).

Changes

• HasSidebar trait: $sidebarWidth and $collapsedSidebarWidth properties changed to string | Closure. Setters accept string | Closure. Getters use $this->evaluate().

Usage

$panel->sidebarWidth(fn (): string => auth()->user()?->prefersWideSidebar() ? '24rem' : '20rem');
$panel->collapsedSidebarWidth(fn (): string => auth()->user()?->isCollapsedSidebarEnabled() ? '4.5rem' : '3rem');

Follows the established Closure pattern from HasTopNavigation, HasSpaMode, HasTopbar.