feat: update frontend for unified order model

Update all frontend templates and JavaScript to use new unified Order model:
- Orders tab: use status field, processing/cancelled values, items array
- Order detail: use snapshot fields, items array, tracking_provider
- JavaScript: update API params (status vs sync_status), orderStats fields
- Tracking modal: use tracking_provider instead of tracking_carrier
- Order items modal: use items array with item_state field

All status mappings:
- pending → pending (unconfirmed)
- processing → confirmed (at least one item available)
- cancelled → declined (all items unavailable)
- shipped → shipped (with tracking)

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

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

docs/implementation/unified-order-view.md

@@ -267,4 +267,9 @@ Each marketplace would use:
 - [x] Letzshop order service updated
 - [x] Letzshop schemas updated
 - [x] API endpoints updated
-- [ ] Frontend updated
+- [x] Frontend updated
+  - [x] Orders tab template (status badges, filters, table)
+  - [x] Order detail page (snapshots, items, tracking)
+  - [x] JavaScript (API params, response handling)
+  - [x] Tracking modal (tracking_provider field)
+  - [x] Order items modal (items array, item_state)