Add click-on-same-month to exit drill-down

Clicking on the already-selected month button now returns to root
view when drilled down, providing a quick way to reset the chart
without using browser back or center click.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app.js

@@ -73,6 +73,16 @@ function navigateForward() {
     }
 }
 
+// Go directly to root level
+function goToRoot() {
+    if (drillDownHistory.length > 0 && historyIndex > 0) {
+        historyIndex = 0;
+        navigateToHistoryState(drillDownHistory[0]);
+        // Update browser history to reflect root state
+        history.replaceState({ drillDown: 0 }, '');
+    }
+}
+
 // Listen for browser back/forward via popstate
 window.addEventListener('popstate', function(e) {
     // If a modal is open, close it and push state back to prevent navigation
@@ -1602,9 +1612,16 @@ function navigateToPath(sunburstData, path) {
 
 // Select and load a specific month
 async function selectMonth(index) {
-    currentMonthIndex = index;
     const month = availableMonths[index];
 
+    // If clicking on the already-selected month while drilled down, reset to root
+    if (index === currentMonthIndex && currentDrillPath.length > 0) {
+        goToRoot();
+        return;
+    }
+
+    currentMonthIndex = index;
+
     // Update hidden select for compatibility
     const select = document.getElementById('month-select');
     select.value = month;