feat: enhance forms with decimal formatting and validation

- Updated CustomersPage.vue to use decimalString for standard discount percent.
- Enhanced IncomingInvoicesPage.vue to format item quantities, unit prices, and tax rates using decimalString.
- Improved ItemsPage.vue with new supplier price management and decimal formatting for prices.
- Modified OrganizationSetupPage.vue to use a dropdown for default tax rates and ensure numeric input for payment days.
- Updated OutgoingInvoicesPage.vue to apply decimal formatting for customer discounts and item details.
- Enhanced PriceImportsPage.vue to include additional fields in the import format.
- Improved PriceRulesPage.vue to use decimal input for markup percentages.
- Updated QuotesPage.vue to apply decimal formatting for customer discounts and item details.
- Enhanced SuppliersPage.vue to use decimal input for standard discount percent.
- Added a new SQL migration to set default unit for items to 'Stck'.
- Introduced format.ts for centralized decimal and currency formatting utilities.

backend/company-migrations/0014_activity_line_sources.sql

@@ -0,0 +1,13 @@
+alter table {schema}.quote_items
+    add column if not exists line_kind text not null default 'item',
+    add column if not exists activity_id uuid references {schema}.activities(id);
+
+alter table {schema}.quote_items
+    alter column item_id drop not null;
+
+alter table {schema}.outgoing_invoice_items
+    add column if not exists line_kind text not null default 'item',
+    add column if not exists activity_id uuid references {schema}.activities(id);
+
+alter table {schema}.outgoing_invoice_items
+    alter column item_id drop not null;