feat: Fill missing dates in charts + no-data states

## Task 1: Fill Missing Dates in Chart Data ✅

**Issue:** Charts only show dates with actual data, causing:
- Gaps in timeline
- Tight/crowded lines on mobile
- Inconsistent X-axis

**Solution:** Backend now fills ALL dates in range with zeros

**Files Updated:**
- `includes/Api/AnalyticsController.php`
  - `calculate_revenue_metrics()` - Revenue chart
  - `calculate_orders_metrics()` - Orders chart
  - `calculate_coupons_metrics()` - Coupons chart

**Implementation:**
```php
// Create data map from query results
$data_map = [];
foreach ($chart_data_raw as $row) {
    $data_map[$row->date] = [...];
}

// Fill ALL dates in range
for ($i = $days - 1; $i >= 0; $i--) {
    $date = date('Y-m-d', strtotime("-{$i} days"));
    if (isset($data_map[$date])) {
        // Use real data
    } else {
        // Fill with zeros
    }
}
```

**Result:**
- ✅ Consistent X-axis with all dates
- ✅ No gaps in timeline
- ✅ Better mobile display (evenly spaced)

---

## Task 2: No-Data States for Charts ✅

**Issue:** Charts show broken/empty state when no data

**Solution:** Show friendly message like Overview does

**Files Updated:**
- `admin-spa/src/routes/Dashboard/Revenue.tsx`
- `admin-spa/src/routes/Dashboard/Orders.tsx`
- `admin-spa/src/routes/Dashboard/Coupons.tsx`

**Implementation:**
```tsx
{chartData.length === 0 || chartData.every(d => d.value === 0) ? (
  <div className="flex items-center justify-center h-[300px]">
    <div className="text-center">
      <Package className="w-12 h-12 text-muted-foreground mx-auto mb-3" />
      <p className="text-muted-foreground font-medium">
        No {type} data available
      </p>
      <p className="text-sm text-muted-foreground mt-1">
        Data will appear once you have {action}
      </p>
    </div>
  </div>
) : (
  <ResponsiveContainer>...</ResponsiveContainer>
)}
```

**Result:**
- ✅ Revenue: "No revenue data available"
- ✅ Orders: "No orders data available"
- ✅ Coupons: "No coupon usage data available"
- ✅ Consistent with Overview page
- ✅ User-friendly empty states

---

## Summary

✅ **Backend:** All dates filled in chart data
✅ **Frontend:** No-data states added to 3 charts
✅ **UX:** Consistent, professional empty states

**Next:** VIP customer settings + mobile chart optimization

admin-spa/src/routes/Dashboard/Coupons.tsx

@@ -1,6 +1,6 @@
 import React, { useState, useMemo } from 'react';
 import { LineChart, Line, XAxis, YAxis, CartesianGrid, Tooltip, ResponsiveContainer, Legend } from 'recharts';
-import { Tag, DollarSign, TrendingUp, ShoppingCart } from 'lucide-react';
+import { Tag, DollarSign, TrendingUp, ShoppingCart, Package } from 'lucide-react';
 import { __ } from '@/lib/i18n';
 import { formatMoney, getStoreCurrency } from '@/lib/currency';
 import { useDashboardPeriod } from '@/hooks/useDashboardPeriod';
@@ -223,8 +223,19 @@ export default function CouponsReport() {
         title={__('Coupon Usage Over Time')}
         description={__('Daily coupon usage and discount amount')}
       >
-        <ResponsiveContainer width="100%" height={300}>
-          <LineChart data={chartData}>
+        {chartData.length === 0 || chartData.every((d: any) => d.uses === 0) ? (
+          <div className="flex items-center justify-center h-[300px]">
+            <div className="text-center">
+              <Package className="w-12 h-12 text-muted-foreground mx-auto mb-3" />
+              <p className="text-muted-foreground font-medium">{__('No coupon usage data available')}</p>
+              <p className="text-sm text-muted-foreground mt-1">
+                {__('Coupon data will appear once customers use coupons')}
+              </p>
+            </div>
+          </div>
+        ) : (
+          <ResponsiveContainer width="100%" height={300}>
+            <LineChart data={chartData}>
             <CartesianGrid strokeDasharray="3 3" className="stroke-muted" />
             <XAxis 
               dataKey="date" 
@@ -283,6 +294,7 @@ export default function CouponsReport() {
             />
           </LineChart>
         </ResponsiveContainer>
+        )}
       </ChartCard>
 
       <ChartCard

admin-spa/src/routes/Dashboard/Orders.tsx

@@ -206,8 +206,19 @@ export default function OrdersAnalytics() {
         title={__('Orders Over Time')}
         description={__('Daily order count and status breakdown')}
       >
-        <ResponsiveContainer width="100%" height={300}>
-          <ComposedChart data={chartData}>
+        {chartData.length === 0 || chartData.every((d: any) => d.orders === 0) ? (
+          <div className="flex items-center justify-center h-[300px]">
+            <div className="text-center">
+              <Package className="w-12 h-12 text-muted-foreground mx-auto mb-3" />
+              <p className="text-muted-foreground font-medium">{__('No orders data available')}</p>
+              <p className="text-sm text-muted-foreground mt-1">
+                {__('Order data will appear once you have orders')}
+              </p>
+            </div>
+          </div>
+        ) : (
+          <ResponsiveContainer width="100%" height={300}>
+            <ComposedChart data={chartData}>
             <defs>
               <linearGradient id="totalOrdersGradient" x1="0" y1="0" x2="0" y2="1">
                 <stop offset="5%" stopColor="#3b82f6" stopOpacity={0.3}/>
@@ -287,6 +298,7 @@ export default function OrdersAnalytics() {
             />
           </ComposedChart>
         </ResponsiveContainer>
+        )}
       </ChartCard>
 
       {/* Two Column Layout */}

admin-spa/src/routes/Dashboard/Revenue.tsx

@@ -1,6 +1,6 @@
 import React, { useState, useMemo } from 'react';
 import { AreaChart, Area, XAxis, YAxis, CartesianGrid, Tooltip, ResponsiveContainer, Legend } from 'recharts';
-import { DollarSign, TrendingUp, TrendingDown, CreditCard, Truck, RefreshCw } from 'lucide-react';
+import { DollarSign, TrendingUp, TrendingDown, CreditCard, Truck, RefreshCw, Package } from 'lucide-react';
 import { __ } from '@/lib/i18n';
 import { formatMoney, getStoreCurrency } from '@/lib/currency';
 import { useDashboardPeriod } from '@/hooks/useDashboardPeriod';
@@ -396,8 +396,19 @@ export default function RevenueAnalytics() {
           </Select>
         }
       >
-        <ResponsiveContainer width="100%" height={300}>
-          <AreaChart data={chartData}>
+        {chartData.length === 0 || chartData.every((d: any) => d.gross === 0 && d.net === 0) ? (
+          <div className="flex items-center justify-center h-[300px]">
+            <div className="text-center">
+              <Package className="w-12 h-12 text-muted-foreground mx-auto mb-3" />
+              <p className="text-muted-foreground font-medium">{__('No revenue data available')}</p>
+              <p className="text-sm text-muted-foreground mt-1">
+                {__('Revenue data will appear once you have completed orders')}
+              </p>
+            </div>
+          </div>
+        ) : (
+          <ResponsiveContainer width="100%" height={300}>
+            <AreaChart data={chartData}>
             <defs>
               <linearGradient id="colorGross" x1="0" y1="0" x2="0" y2="1">
                 <stop offset="5%" stopColor="#3b82f6" stopOpacity={0.3}/>
@@ -467,6 +478,7 @@ export default function RevenueAnalytics() {
             />
           </AreaChart>
         </ResponsiveContainer>
+        )}
       </ChartCard>
 
       {/* Revenue Breakdown Tables */}