feat: integer cents money handling, order page fixes, and vendor filter persistence

Money Handling Architecture:
- Store all monetary values as integer cents (€105.91 = 10591)
- Add app/utils/money.py with Money class and conversion helpers
- Add static/shared/js/money.js for frontend formatting
- Update all database models to use _cents columns (Product, Order, etc.)
- Update CSV processor to convert prices to cents on import
- Add Alembic migration for Float to Integer conversion
- Create .architecture-rules/money.yaml with 7 validation rules
- Add docs/architecture/money-handling.md documentation

Order Details Page Fixes:
- Fix customer name showing 'undefined undefined' - use flat field names
- Fix vendor info empty - add vendor_name/vendor_code to OrderDetailResponse
- Fix shipping address using wrong nested object structure
- Enrich order detail API response with vendor info

Vendor Filter Persistence Fixes:
- Fix orders.js: restoreSavedVendor now sets selectedVendor and filters
- Fix orders.js: init() only loads orders if no saved vendor to restore
- Fix marketplace-letzshop.js: restoreSavedVendor calls selectVendor()
- Fix marketplace-letzshop.js: clearVendorSelection clears TomSelect dropdown
- Align vendor selector placeholder text between pages

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

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

.architecture-rules/_main.yaml

@@ -28,6 +28,9 @@ principles:
   - name: "Consistent Naming"
     description: "API files: plural, Services: singular+service, Models: singular"
 
+  - name: "Integer Cents Money"
+    description: "All monetary values stored as integer cents. Convert to euros only at display layer. See docs/architecture/money-handling.md"
+
 # ============================================================================
 # RULE FILE INCLUDES
 # ============================================================================
@@ -43,6 +46,7 @@ includes:
   - frontend.yaml
   - language.yaml
   - quality.yaml
+  - money.yaml
 
 # ============================================================================
 # VALIDATION SEVERITY LEVELS
@@ -100,3 +104,4 @@ documentation:
   frontend: "docs/frontend/overview.md"
   contributing: "docs/development/contributing.md"
   code_quality: "docs/development/code-quality.md"
+  money_handling: "docs/architecture/money-handling.md"