feat: enhance UI and functionality for price rules, quotes, and suppliers

- Updated PriceRulesPage.vue to replace "Neu" button with a more intuitive "+" button for creating new rules.
- Enhanced QuotesPage.vue with improved handling of quote lines, including read-only states, unique line validation, and better formatting for monetary values.
- Added accordion sections for better organization of quote details and improved user experience.
- Updated SuppliersPage.vue to replace "Neu" button with a "+" button for adding new suppliers.
- Introduced new database migrations to add price category and default sales price fields to activities, and to include 'invoice_created' status in quotes.

backend/company-migrations/0017_quote_invoice_created_status.sql

@@ -0,0 +1,7 @@
+alter table {schema}.quotes
+    drop constraint if exists quotes_status_valid;
+
+alter table {schema}.quotes
+    add constraint quotes_status_valid check (
+        status in ('draft', 'sent', 'accepted', 'rejected', 'expired', 'cancelled', 'invoice_created')
+    );