Default issue date field to today's date

Prefill 'Datum vystaveni' with the current local date when opening a
new invoice, instead of leaving it blank. Also switch date formatting
to use local date components instead of toISOString() to avoid UTC
timezone shifting the displayed date.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

app/src/static/script.js

@@ -207,7 +207,11 @@ const getClientLabel = (client) => [
 
 const formatCzechDate = (dateValue) => new Date(`${dateValue}T00:00:00`).toLocaleDateString("cs-CZ");
 
-const toDateInputValue = (date) => date.toISOString().slice(0, 10);
+const toDateInputValue = (date) => [
+    date.getFullYear(),
+    String(date.getMonth() + 1).padStart(2, "0"),
+    String(date.getDate()).padStart(2, "0")
+].join("-");
 
 const getIssueDate = () => issueDateInput.value || toDateInputValue(new Date());
 
@@ -365,7 +369,7 @@ const clearInvoiceEditor = () => {
     currentInvoiceId = null;
     invoiceNumberDisplay.textContent = "Nová faktura";
     invoiceSaveStatus.textContent = "";
-    issueDateInput.value = "";
+    issueDateInput.value = toDateInputValue(new Date());
     dueDateInput.value = "";
     writeProfileFields("odberatel", {});
     clientSelect.value = "";