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.

web-frontend/src/format.ts

@@ -0,0 +1,62 @@
+const decimalFormatter = new Intl.NumberFormat("de-DE", {
+  minimumFractionDigits: 2,
+  maximumFractionDigits: 4
+});
+
+const currencyFormatter = new Intl.NumberFormat("de-DE", {
+  style: "currency",
+  currency: "EUR",
+  minimumFractionDigits: 2,
+  maximumFractionDigits: 4
+});
+
+export const unitOptions = ["Stck", "kg", "g", "L", "mg", "ml", "qm", "m", "cm", "mm"];
+export const taxRateOptions = ["0", "7", "19"];
+
+export function formatDecimal(value: string | number | null | undefined): string {
+  const number = toNumber(value);
+  return number === null ? "" : decimalFormatter.format(number);
+}
+
+export function formatEuro(value: string | number | null | undefined): string {
+  const number = toNumber(value);
+  return number === null ? "-" : currencyFormatter.format(number);
+}
+
+export function normalizeDecimal(value: string | number | null | undefined): string | null {
+  if (value === null || value === undefined || value === "") return null;
+  const text = String(value).trim();
+  if (!text) return null;
+  const compact = text.replace(/\s/g, "");
+  if (compact.includes(",")) return compact.replace(/\./g, "").replace(",", ".");
+  const dotCount = (compact.match(/\./g) ?? []).length;
+  if (dotCount > 1) return compact.replace(/\./g, "");
+  if (/^-?\d{1,3}\.\d{3}$/.test(compact)) return compact.replace(".", "");
+  return compact;
+}
+
+export function formatDecimalInput<T extends Record<string, unknown>>(target: T, key: keyof T) {
+  const normalized = normalizeDecimal(target[key] as string | number | null | undefined);
+  target[key] = (normalized === null ? null : formatDecimal(normalized)) as T[keyof T];
+}
+
+export function decimalString(value: string | number | null | undefined, fallback = "0"): string {
+  return normalizeDecimal(value) ?? fallback;
+}
+
+export function normalizeTaxRate(value: string | number | null | undefined, fallback = "19"): string {
+  const normalized = normalizeDecimal(value);
+  if (normalized === null) return fallback;
+  const number = Number(normalized);
+  if (!Number.isFinite(number)) return normalized;
+  const compact = String(number);
+  return taxRateOptions.includes(compact) ? compact : normalized;
+}
+
+function toNumber(value: string | number | null | undefined): number | null {
+  if (value === null || value === undefined || value === "") return null;
+  const normalized = normalizeDecimal(value);
+  if (normalized === null) return null;
+  const number = Number(normalized);
+  return Number.isFinite(number) ? number : null;
+}