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/0016_activity_price_fields.sql

@@ -0,0 +1,17 @@
+alter table {schema}.activities
+    add column if not exists price_category text not null default 'h',
+    add column if not exists default_sales_price numeric(14, 4);
+
+alter table {schema}.activities
+    drop constraint if exists activities_price_category_valid;
+
+alter table {schema}.activities
+    add constraint activities_price_category_valid check (price_category in ('h', 'tag', 'pauschal'));
+
+alter table {schema}.activities
+    drop constraint if exists activities_default_sales_price_non_negative;
+
+alter table {schema}.activities
+    add constraint activities_default_sales_price_non_negative check (
+        default_sales_price is null or default_sales_price >= 0
+    );