feat: Implement A4 invoice layout and hide Label for virtual orders

Invoice:
- Enhanced A4-ready layout with proper structure
- Store header with invoice number
- Billing/shipping address sections
- Styled items table with alternating rows
- Totals summary with conditional display
- Thank you footer

Label:
- Label button now hidden for virtual-only orders
- Uses existing isVirtualOnly detection

Print CSS:
- Added @page A4 size directive
- Print-color-adjust for background colors
- 20mm padding for proper margins

Documentation:
- Updated subscription module plan (comprehensive)
- Updated affiliate module plan (comprehensive)
- Created shipping label standardization plan

.agent/plans/shipping-label.md

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+# Shipping Label Plan
+
+## Overview
+Standardized waybill data structure for shipping label generation.
+
+## Problem
+- Different shipping carrier addons (JNE, JNT, SiCepat, etc.) store data differently
+- No standard structure for label generation
+- Label button needs waybill data to function
+
+## Proposed Solution
+
+### 1. Standardized Meta Key
+Order meta: `_shipping_waybill`
+
+### 2. Data Structure
+```json
+{
+  "tracking_number": "JNE123456789",
+  "carrier": "jne",
+  "carrier_name": "JNE Express",
+  "service": "REG",
+  "estimated_days": 3,
+  "sender": {
+    "name": "Store Name",
+    "address": "Full address line 1",
+    "city": "Jakarta",
+    "postcode": "12345",
+    "phone": "08123456789"
+  },
+  "recipient": {
+    "name": "Customer Name", 
+    "address": "Full address line 1",
+    "city": "Bandung",
+    "postcode": "40123",
+    "phone": "08987654321"
+  },
+  "package": {
+    "weight": "1.5",
+    "weight_unit": "kg",
+    "dimensions": "20x15x10",
+    "dimensions_unit": "cm"
+  },
+  "label_url": null,
+  "barcode": "JNE123456789",
+  "barcode_type": "128",
+  "created_at": "2026-01-05T12:00:00+07:00"
+}
+```
+
+### 3. Addon Integration Contract
+
+Shipping addons MUST:
+1. Call `update_post_meta($order_id, '_shipping_waybill', $waybill_data)`
+2. Use the standard structure above
+3. Set `label_url` if carrier provides downloadable PDF
+4. Set `barcode` for local label generation
+
+### 4. Label Button Behavior
+1. Check if `_shipping_waybill` meta exists on order
+2. If `label_url` → open carrier's PDF
+3. Otherwise → generate printable label from meta data
+
+### 5. UI Behavior
+- Label button hidden if order is virtual-only
+- Label button shows "Generate Label" if no waybill yet
+- Label button shows "Print Label" if waybill exists
+
+## API Endpoint (Future)
+```
+POST /woonoow/v1/orders/{id}/generate-waybill
+- Calls shipping carrier API
+- Stores waybill in standardized format
+- Returns waybill data
+
+GET /woonoow/v1/orders/{id}/waybill
+- Returns current waybill data
+```
+
+## Implementation Priority
+1. Define standard structure (this document)
+2. Implement Label UI conditional logic
+3. Create waybill API endpoint
+4. Document for addon developers