docs: Update PROJECT_BRIEF with Settings architecture philosophy

📝 Changes:
- Added Phase 4.5: Settings SPA & Setup Wizard
- Added Section 5: Settings Architecture Philosophy
- Documented 'better wardrobe' approach
- Clarified WooCommerce bone structure respect
- Defined compatibility stance

🎯 Key Principles:
- Read WC structure (don't create parallel system)
- Transform & simplify (better UX)
- Enhance performance (like Orders: 30s → 1-2s)
- Respect ecosystem (auto-support WC-compliant addons)
- No new hooks (listen to WC hooks)

🎨 UI Strategy:
- Generic form builder (standard)
- Custom components (popular gateways)
- Redirect to WC (complex/non-standard)
- Multi-page forms (20+ fields)

Compatibility: 'If it works in WC, it works in WooNooW'

admin-spa/src/routes/Settings/components/SettingsLayout.tsx

@@ -58,8 +58,8 @@ export function SettingsLayout({
         </div>
       )}
 
-      {/* Content - Full width like Orders/Dashboard */}
-      <div className="px-4 py-8 w-full">
+      {/* Content */}
+      <div className="container max-w-5xl mx-auto px-4 py-8">
         {!onSave && (
           <div className="mb-8">
             <h1 className="text-2xl font-bold tracking-tight">{title}</h1>
@@ -68,12 +68,13 @@ export function SettingsLayout({
             )}
           </div>
         )}
+
         {isLoading ? (
           <div className="flex items-center justify-center py-12">
             <Loader2 className="h-8 w-8 animate-spin text-muted-foreground" />
           </div>
         ) : (
-          <div className="space-y-6 max-w-6xl">{children}</div>
+          <div className="space-y-6">{children}</div>
         )}
       </div>
     </div>