Fix monthly PDF export to include only the selected month

The month dropdown carried only the month (e.g. "07") while the year was
a separate select; the export required month AND year, so a month-only
selection fell through to exporting every application. Encode the year in
the month option value ("YYYY-MM"), parse it client-side to always send
the exact month+year, and harden /api/export so a month can never fall
through to "export all".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/js/main.js

@@ -251,9 +251,21 @@ function openPdfExportModal() {
 function generatePDF(event) {
     event.preventDefault();
     
-    const month = document.getElementById('pdfMonth').value;
-    const year = document.getElementById('pdfYear').value;
-    
+    // The month option encodes its year ("YYYY-MM"), so a chosen month always
+    // exports exactly that month of that year, regardless of the year select.
+    const monthRaw = document.getElementById('pdfMonth').value;
+    let year = document.getElementById('pdfYear').value;
+    let month = '';
+    if (monthRaw) {
+        if (monthRaw.indexOf('-') !== -1) {
+            const parts = monthRaw.split('-');
+            year = parts[0];
+            month = parts[1];
+        } else {
+            month = monthRaw;
+        }
+    }
+
     // Fetch data for PDF
     let url = '/api/export?';
     const params = [];