feat: add vendor invoice management UI and comprehensive tests

UI Components:
- Add vendor invoices page route in vendor_pages.py
- Create invoices.html template with stats cards, invoice table,
  settings tab, and create invoice modal
- Add invoices.js Alpine.js component for CRUD operations,
  PDF download, and settings management
- Add Invoices link to vendor sidebar in Sales section

Unit Tests (35 tests):
- VAT calculation (EU rates, regimes, labels)
- Invoice settings CRUD and number generation
- Invoice retrieval, listing, and pagination
- Status management and validation
- Statistics calculation

Integration Tests (34 tests):
- Settings API endpoints (GET/POST/PUT)
- Stats API endpoint
- Invoice list with filtering and pagination
- Invoice detail retrieval
- Invoice creation from orders
- Status update transitions
- PDF generation endpoints
- Authentication/authorization checks

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routes/vendor_pages.py

@@ -353,6 +353,33 @@ async def vendor_letzshop_page(
     )
 
 
+# ============================================================================
+# INVOICES
+# ============================================================================
+
+
+@router.get(
+    "/{vendor_code}/invoices", response_class=HTMLResponse, include_in_schema=False
+)
+async def vendor_invoices_page(
+    request: Request,
+    vendor_code: str = Path(..., description="Vendor code"),
+    current_user: User = Depends(get_current_vendor_from_cookie_or_header),
+):
+    """
+    Render invoices management page.
+    JavaScript loads invoices via API.
+    """
+    return templates.TemplateResponse(
+        "vendor/invoices.html",
+        {
+            "request": request,
+            "user": current_user,
+            "vendor_code": vendor_code,
+        },
+    )
+
+
 # ============================================================================
 # TEAM MANAGEMENT
 # ============================================================================