v1.6.8: remove offer exchange rate, add invoice item description, offer filters, table animations

- Remove exchange_rate and exchange_rate_date from quotations (schema, service, PDF, form)
- Add item_description field to invoice_items (schema, migration, service, form, PDF)
- Add offer status/customer/order filters with tab-based UI
- Clean up offer statuses to active/ordered/invalidated
- Allow invalidate action for non-ordered offers (not just expired)
- Add fade animations on offers and invoices table data changes

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/services/orders.service.ts

@@ -206,7 +206,6 @@ export async function createOrderFromQuotation(
         language: quotation.language || "cs",
         vat_rate: quotation.vat_rate ?? 21.0,
         apply_vat: quotation.apply_vat ?? true,
-        exchange_rate: quotation.exchange_rate ?? 1.0,
         scope_title: quotation.scope_title,
         scope_description: quotation.scope_description,
         attachment_data: attachmentBuffer