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/providers/forms/BasicFormFields.tsx

@@ -80,18 +80,11 @@ export function BasicFormFields({ form }: BasicFormFieldsProps) {
                     <ArrowLeft className="h-5 w-5" />
                   </Button>
                 </DialogClose>
-                <div className="space-y-1">
-                  <p className="text-lg font-semibold leading-tight">
-                    {t("providerIcon.selectIcon", {
-                      defaultValue: "选择图标",
-                    })}
-                  </p>
-                  <p className="text-sm text-muted-foreground">
-                    {t("providerIcon.selectDescription", {
-                      defaultValue: "为供应商选择一个图标",
-                    })}
-                  </p>
-                </div>
+                <p className="text-lg font-semibold leading-tight">
+                  {t("providerIcon.selectIcon", {
+                    defaultValue: "选择图标",
+                  })}
+                </p>
               </div>
               <div className="flex-1 overflow-y-auto px-6 py-6">
                 <div className="space-y-6 max-w-5xl mx-auto w-full">