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

includes/Api/AnalyticsController.php

@@ -534,20 +534,39 @@ class AnalyticsController {
 		$total_tax = 0;
 		$total_refunds = 0;
 		$order_count = 0;
-		$chart_data = [];
 		
+		// Create a map of existing data by date
+		$data_map = [];
 		foreach ($chart_data_raw as $row) {
-			$gross = round(floatval($row->gross), 2);
-			$net = round(floatval($row->net), 2);
-			$tax = round(floatval($row->tax), 2);
+			$data_map[$row->date] = [
+				'gross' => round(floatval($row->gross), 2),
+				'net' => round(floatval($row->net), 2),
+				'tax' => round(floatval($row->tax), 2),
+			];
+		}
+		
+		// Fill in ALL dates in the range (including dates with no data)
+		$chart_data = [];
+		for ($i = $days - 1; $i >= 0; $i--) {
+			$date = date('Y-m-d', strtotime("-{$i} days"));
+			
+			if (isset($data_map[$date])) {
+				$gross = $data_map[$date]['gross'];
+				$net = $data_map[$date]['net'];
+				$tax = $data_map[$date]['tax'];
+			} else {
+				// No data for this date, fill with zeros
+				$gross = 0.00;
+				$net = 0.00;
+				$tax = 0.00;
+			}
 			
 			$total_gross += $gross;
 			$total_net += $net;
 			$total_tax += $tax;
 			
-			// Format for frontend
 			$chart_data[] = [
-				'date' => $row->date,
+				'date' => $date,
 				'gross' => $gross,
 				'net' => $net,
 				'tax' => $tax,
@@ -930,11 +949,10 @@ class AnalyticsController {
 			LIMIT %d
 		", $days));
 		
-		// Format chart data
-		$formatted_chart = [];
+		// Format chart data - Create a map of existing data by date
+		$data_map = [];
 		foreach ($chart_data as $row) {
-			$formatted_chart[] = [
-				'date' => $row->date,
+			$data_map[$row->date] = [
 				'orders' => intval($row->orders),
 				'completed' => intval($row->completed),
 				'processing' => intval($row->processing),
@@ -945,6 +963,28 @@ class AnalyticsController {
 			];
 		}
 		
+		// Fill in ALL dates in the range (including dates with no data)
+		$formatted_chart = [];
+		for ($i = $days - 1; $i >= 0; $i--) {
+			$date = date('Y-m-d', strtotime("-{$i} days"));
+			
+			if (isset($data_map[$date])) {
+				$formatted_chart[] = array_merge(['date' => $date], $data_map[$date]);
+			} else {
+				// No data for this date, fill with zeros
+				$formatted_chart[] = [
+					'date' => $date,
+					'orders' => 0,
+					'completed' => 0,
+					'processing' => 0,
+					'pending' => 0,
+					'cancelled' => 0,
+					'refunded' => 0,
+					'failed' => 0,
+				];
+			}
+		}
+		
 		// Calculate average order value and rates
 		$avg_order_value = $total_orders > 0 ? round($total_revenue / $total_orders, 2) : 0;
 		
@@ -1189,21 +1229,39 @@ class AnalyticsController {
 			ORDER BY date ASC
 		", $days));
 		
-		$formatted_chart = [];
+		// Create a map of existing data by date
+		$data_map = [];
 		$revenue_with_coupons = 0;
 		
 		foreach ($usage_chart as $row) {
 			$revenue = round(floatval($row->revenue), 2);
 			$revenue_with_coupons += $revenue;
 			
-			$formatted_chart[] = [
-				'date' => $row->date,
+			$data_map[$row->date] = [
 				'uses' => intval($row->uses),
 				'discount' => round(floatval($row->discount), 2),
 				'revenue' => $revenue,
 			];
 		}
 		
+		// Fill in ALL dates in the range (including dates with no data)
+		$formatted_chart = [];
+		for ($i = $days - 1; $i >= 0; $i--) {
+			$date = date('Y-m-d', strtotime("-{$i} days"));
+			
+			if (isset($data_map[$date])) {
+				$formatted_chart[] = array_merge(['date' => $date], $data_map[$date]);
+			} else {
+				// No data for this date, fill with zeros
+				$formatted_chart[] = [
+					'date' => $date,
+					'uses' => 0,
+					'discount' => 0.00,
+					'revenue' => 0.00,
+				];
+			}
+		}
+		
 		$avg_discount_per_order = $total_uses > 0 ? round($total_discount / $total_uses, 2) : 0;
 		
 		return [