Add coupons, newsletter, country-based shipping, sale prices, USP bar

- Gutscheincodes: DB-Verwaltung (%, €-Rabatt, Mindestbestellwert, Ablauf),
  Eingabefeld im Checkout, Admin unter Marketing → Gutscheine
- Newsletter: Double-Opt-In per E-Mail, Formular im Footer,
  Admin-Liste mit CSV-Export
- Länderzonen-Versand: DE / DACH / EU / WORLD, Versandkosten nach
  Lieferland dynamisch berechnet, Coupon-Rabatt in Bestellung gespeichert
- Sale-Preise: sale_price auf Produkten, durchgestrichener UVP auf
  Produktkarte, Sonderangebote-Sektion auf Startseite
- USP-Leiste auf Startseite: Versand, Zahlung, Rückgabe, Service

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/Product.php

@@ -8,9 +8,19 @@ use Illuminate\Database\Eloquent\Relations\HasMany;
 
 class Product extends Model
 {
-    protected $fillable = ['category_id','name','slug','description','base_price','tax_rate','weight','active','featured','track_stock','sort_order'];
+    protected $fillable = ['category_id','name','slug','description','base_price','sale_price','tax_rate','weight','active','featured','track_stock','sort_order'];
 
-    protected $casts = ['active' => 'boolean', 'featured' => 'boolean', 'track_stock' => 'boolean', 'base_price' => 'decimal:2', 'tax_rate' => 'decimal:2'];
+    protected $casts = ['active' => 'boolean', 'featured' => 'boolean', 'track_stock' => 'boolean', 'base_price' => 'decimal:2', 'sale_price' => 'decimal:2', 'tax_rate' => 'decimal:2'];
+
+    public function isOnSale(): bool
+    {
+        return $this->sale_price !== null && (float)$this->sale_price < (float)$this->base_price;
+    }
+
+    public function getEffectivePriceAttribute(): float
+    {
+        return $this->isOnSale() ? (float)$this->sale_price : (float)$this->base_price;
+    }
 
     public function category(): BelongsTo
     {