refactor(ui): unify layout system with 60rem max-width and consistent spacing

- Standardize container max-width across all panels:
  * Replace max-w-4xl and max-w-5xl with unified max-w-[60rem]
  * Apply to SettingsPage, UnifiedMcpPanel, SkillsPage, and FullScreenPanel
  * Ensures consistent reading width and visual balance on wide screens

- Optimize padding hierarchy and structure:
  * Move px-6 from parent elements to content containers
  * FullScreenPanel: Add max-w-[60rem] wrapper to header, content, and footer
  * Add border separators (border-t/border-b) to header and footer sections
  * Consolidate nested padding in MCP, Skills, and Prompts panels
  * Remove redundant padding layers for cleaner CSS

- Simplify component styling:
  * MCP list items: Replace card-based layout with modern group-based design
  * Remove unnecessary wrapper divs and flatten DOM structure
  * Update card hover effects with smooth transitions
  * Simplify icon selection dialog (remove description text in BasicFormFields)

- Benefits:
  * Consistent 60rem max-width provides optimal readability
  * Unified spacing rules reduce maintenance complexity
  * Cleaner component hierarchy improves performance
  * Better responsive behavior across different screen sizes
  * More cohesive visual design language throughout the app

This creates a maintainable, scalable design system foundation.

src/components/settings/SettingsPage.tsx

@@ -168,7 +168,7 @@ export function SettingsPage({
   const isBusy = useMemo(() => isLoading && !settings, [isLoading, settings]);
 
   return (
-    <div className="mx-auto max-w-5xl flex flex-col h-[calc(100vh-8rem)]">
+    <div className="mx-auto max-w-[60rem] flex flex-col h-[calc(100vh-8rem)] px-6">
       {isBusy ? (
         <div className="flex flex-1 items-center justify-center">
           <Loader2 className="h-8 w-8 animate-spin text-muted-foreground" />