refactor(layout): standardize max-width to 60rem and optimize padding structure

- Unify container max-width across components:
  * Replace max-w-4xl with max-w-[60rem] in App.tsx provider list
  * Replace max-w-5xl with max-w-[60rem] in PromptPanel
  * Move padding from header element to inner container for consistent spacing
- Optimize padding hierarchy:
  * Remove px-6 from header element, add to inner header container
  * Remove px-6 from main element, keep it only in provider list container
  * Consolidate PromptPanel padding: move px-6 from nested divs to outer container
  * Remove redundant pl-1 sm:pl-2 from logo/title area
- Benefits:
  * Consistent 60rem max-width provides better readability on wide screens
  * Simplified padding structure reduces CSS complexity
  * Cleaner responsive behavior with unified spacing rules

This creates a more maintainable and visually consistent layout system.

src/components/prompts/PromptPanel.tsx

@@ -80,8 +80,8 @@ const PromptPanel = React.forwardRef<PromptPanelHandle, PromptPanelProps>(
     const enabledPrompt = promptEntries.find(([_, p]) => p.enabled);
 
     return (
-      <div className="mx-auto max-w-5xl flex flex-col h-[calc(100vh-8rem)]">
-        <div className="flex-shrink-0 px-6 py-4 glass rounded-xl border border-white/10 mb-4">
+      <div className="mx-auto max-w-[60rem] flex flex-col h-[calc(100vh-8rem)] px-6">
+        <div className="flex-shrink-0 py-4 glass rounded-xl border border-white/10 mb-4 px-6">
           <div className="text-sm text-muted-foreground">
             {t("prompts.count", { count: promptEntries.length })} ·{" "}
             {enabledPrompt
@@ -90,7 +90,7 @@ const PromptPanel = React.forwardRef<PromptPanelHandle, PromptPanelProps>(
           </div>
         </div>
 
-        <div className="flex-1 overflow-y-auto px-6 pb-16">
+        <div className="flex-1 overflow-y-auto pb-16">
           {loading ? (
             <div className="text-center py-12 text-gray-500 dark:text-gray-400">
               {t("prompts.loading")}