Make invoice due date optional with 14-day default

Datum vystaveni defaults to today and Datum splatnosti defaults to
14 days after the issue date. Both fields are now optional date
inputs and only override the default when a user enters a custom
value. Applies to the invoice editor and generated PDF.

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

app/src/app.js

@@ -445,6 +445,20 @@ app.delete('/api/invoices', requireAuth, async (req, res, next) => {
 });
 
 const pdfText = (value) => String(value || 'Neuvedeno');
+const formatCzechDate = (value) => {
+    const date = new Date(`${value}T00:00:00`);
+    return Number.isNaN(date.getTime()) ? '' : date.toLocaleDateString('cs-CZ');
+};
+const toDateInputValue = (date) => date.toISOString().slice(0, 10);
+const resolveIssueDate = (issueDate) => issueDate || toDateInputValue(new Date());
+const resolveDueDate = (dueDate, issueDate) => {
+    if (dueDate) {
+        return dueDate;
+    }
+    const due = new Date(`${resolveIssueDate(issueDate)}T00:00:00`);
+    due.setDate(due.getDate() + 14);
+    return toDateInputValue(due);
+};
 const formatCzechNumber = (value) => Number(value || 0).toLocaleString('cs-CZ', {
     minimumFractionDigits: 2,
     maximumFractionDigits: 2
@@ -518,13 +532,25 @@ const renderInvoicePdf = async (document, invoice) => {
     const partyWidth = (pageWidth - 24) / 2;
     const customerX = document.page.margins.left + partyWidth + 24;
 
+    const issueDate = resolveIssueDate(data.issueDate);
+    const dueDate = resolveDueDate(data.dueDate, data.issueDate);
+
     const headerY = document.y;
     document.fillColor(navy).font(boldFont).fontSize(25).text('Faktura', document.page.margins.left, headerY);
     document.fillColor(navy).font(boldFont).fontSize(16).text(invoice.invoice_number, customerX, headerY, {
         width: partyWidth,
         align: 'right'
     });
-    document.y = headerY + 40;
+    document.fillColor(bodyText).font(regularFont).fontSize(9)
+        .text(`Datum vystavení: ${formatCzechDate(issueDate)}`, customerX, headerY + 22, {
+            width: partyWidth,
+            align: 'right'
+        })
+        .text(`Datum splatnosti: ${formatCzechDate(dueDate)}`, customerX, headerY + 35, {
+            width: partyWidth,
+            align: 'right'
+        });
+    document.y = headerY + 52;
 
     const partyTop = document.y;
     const drawParty = (x, title, profile, name) => {