feat: Implement Phase 1 Shopify-inspired settings (Store, Payments, Shipping)

✨ Features:
- Store Details page with live currency preview
- Payments page with visual provider cards and test mode
- Shipping & Delivery page with zone cards and local pickup
- Shared components: SettingsLayout, SettingsCard, SettingsSection, ToggleField

🎨 UI/UX:
- Card-based layouts (not boring forms)
- Generous whitespace and visual hierarchy
- Toast notifications using sonner (reused from Orders)
- Sticky save button at top
- Mobile-responsive design

🔧 Technical:
- Installed ESLint with TypeScript support
- Fixed all lint errors (0 errors)
- Phase 1 files have zero warnings
- Used existing toast from sonner (not reinvented)
- Updated routes in App.tsx

📝 Files Created:
- Store.tsx (currency preview, address, timezone)
- Payments.tsx (provider cards, manual methods)
- Shipping.tsx (zone cards, rates, local pickup)
- SettingsLayout.tsx, SettingsCard.tsx, SettingsSection.tsx, ToggleField.tsx

Phase 1 complete: 18-24 hours estimated work

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

@@ -0,0 +1,23 @@
+import React from 'react';
+import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card';
+
+interface SettingsCardProps {
+  title: string;
+  description?: string;
+  children: React.ReactNode;
+  className?: string;
+}
+
+export function SettingsCard({ title, description, children, className = '' }: SettingsCardProps) {
+  return (
+    <Card className={className}>
+      <CardHeader>
+        <CardTitle>{title}</CardTitle>
+        {description && <CardDescription>{description}</CardDescription>}
+      </CardHeader>
+      <CardContent className="space-y-4">
+        {children}
+      </CardContent>
+    </Card>
+  );
+}