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/SettingsLayout.tsx

@@ -0,0 +1,82 @@
+import React, { useState } from 'react';
+import { Button } from '@/components/ui/button';
+import { Loader2 } from 'lucide-react';
+
+interface SettingsLayoutProps {
+  title: string;
+  description?: string;
+  children: React.ReactNode;
+  onSave?: () => Promise<void>;
+  saveLabel?: string;
+  isLoading?: boolean;
+}
+
+export function SettingsLayout({
+  title,
+  description,
+  children,
+  onSave,
+  saveLabel = 'Save changes',
+  isLoading = false,
+}: SettingsLayoutProps) {
+  const [isSaving, setIsSaving] = useState(false);
+
+  const handleSave = async () => {
+    if (!onSave) return;
+    setIsSaving(true);
+    try {
+      await onSave();
+    } finally {
+      setIsSaving(false);
+    }
+  };
+
+  return (
+    <div className="min-h-screen bg-background">
+      {/* Sticky Header with Save Button */}
+      {onSave && (
+        <div className="sticky top-0 z-10 border-b bg-background/95 backdrop-blur supports-[backdrop-filter]:bg-background/60">
+          <div className="container max-w-5xl mx-auto px-4 py-3 flex items-center justify-between">
+            <div>
+              <h1 className="text-lg font-semibold">{title}</h1>
+            </div>
+            <Button
+              onClick={handleSave}
+              disabled={isSaving || isLoading}
+              size="sm"
+            >
+              {isSaving ? (
+                <>
+                  <Loader2 className="mr-2 h-4 w-4 animate-spin" />
+                  Saving...
+                </>
+              ) : (
+                saveLabel
+              )}
+            </Button>
+          </div>
+        </div>
+      )}
+
+      {/* 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>
+            {description && (
+              <p className="text-muted-foreground mt-2">{description}</p>
+            )}
+          </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">{children}</div>
+        )}
+      </div>
+    </div>
+  );
+}