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/admin/offers.css

@@ -627,3 +627,33 @@
   margin-top: 0.2rem;
   opacity: 0.8;
 }
+
+/* Filters */
+.admin-filters {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 1rem;
+  align-items: flex-start;
+}
+
+.admin-filter-group {
+  display: flex;
+  flex-direction: column;
+  gap: 0.35rem;
+}
+
+.admin-filter-label {
+  font-size: 0.75rem;
+  font-weight: 600;
+  color: var(--text-tertiary);
+  text-transform: uppercase;
+  letter-spacing: 0.03em;
+}
+
+.admin-filter-group .admin-tabs {
+  flex-wrap: wrap;
+}
+
+.admin-filter-group .admin-form-select {
+  min-width: 10rem;
+}