feat: add email notifications, order references, stock badges, and sale flags

- Add customer email field and PayPal payment reminder emails with order reference (HEL-YEAR-ID format)
- Display stock availability with color-coded badges (available/low/unavailable)
- Add sale/clearance flag with animated red badge overlay
- Implement automatic fallback placeholder for missing/broken product images
- Add email column to order management view

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

wawi/static/style.css

@@ -198,25 +198,6 @@ input[type="text"], input[type="number"] {
 }
 .to-top:hover { transform: translateY(-2px); }
 
-.badge {
-  display: inline-block;
-  padding: 4px 10px;
-  border-radius: 4px;
-  font-size: 13px;
-  font-weight: 600;
-  white-space: nowrap;
-}
-.badge.success {
-  background: rgba(123, 213, 141, 0.2);
-  color: var(--ok);
-  border: 1px solid rgba(123, 213, 141, 0.4);
-}
-.badge.warning {
-  background: rgba(243, 213, 42, 0.2);
-  color: var(--accent);
-  border: 1px solid rgba(243, 213, 42, 0.4);
-}
-
 @media (max-width: 720px) {
   .actions { display: flex; gap: 6px; flex-wrap: wrap; }
   .btn.small { padding: 6px 8px; }