Growth charts: label each percentile curve (3rd, 50th, 97th, …)

Adds a Chart.js plugin that draws the percentile label at the right
end of each reference line, matching the convention on printed
WHO/CDC growth charts. Lets clinicians identify lines at a glance
without using the legend. Canvas gets 30px right-padding so the
labels don't get clipped.

public/js/calculators.js

@@ -811,6 +811,34 @@
 
     var _chartInstances = {};
 
+    // Chart.js plugin — draws the percentile label (e.g. "50th") at the
+    // right end of each percentile curve so the lines are identifiable
+    // without a legend. Standard on printed WHO/CDC growth charts.
+    var percentileLabelPlugin = {
+      id: 'percentileLabels',
+      afterDatasetsDraw: function(chart) {
+        var ctx = chart.ctx;
+        chart.data.datasets.forEach(function(ds, i) {
+          // Only the percentile line datasets (labels look like "50th")
+          if (!ds.label || !/^\d+th$/.test(ds.label)) return;
+          var meta = chart.getDatasetMeta(i);
+          if (!meta || meta.hidden || !meta.data || !meta.data.length) return;
+          var last = meta.data[meta.data.length - 1];
+          if (!last) return;
+          ctx.save();
+          ctx.font = '600 10px -apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif';
+          ctx.fillStyle = typeof ds.borderColor === 'string' ? ds.borderColor : '#555';
+          ctx.textAlign = 'left';
+          ctx.textBaseline = 'middle';
+          // Solidify alpha so labels remain readable even when the line is faint
+          var m = ctx.fillStyle.match(/rgba?\((\d+),\s*(\d+),\s*(\d+)/);
+          if (m) ctx.fillStyle = 'rgb(' + m[1] + ',' + m[2] + ',' + m[3] + ')';
+          ctx.fillText(ds.label, last.x + 4, last.y);
+          ctx.restore();
+        });
+      }
+    };
+
     function renderGrowthChart(canvasId, containerId, lmsTable, patientX, patientY, xLabel, yLabel, extraDatasets) {
       if (typeof Chart === 'undefined') return; // Chart.js not loaded yet
 
@@ -841,11 +869,16 @@
       _chartInstances[canvasId] = new Chart(document.getElementById(canvasId), {
         type: 'line',
         data: { datasets: datasets },
+        plugins: [percentileLabelPlugin],
         options: {
           responsive: true,
           maintainAspectRatio: true,
           aspectRatio: 1.6,
           animation: { duration: 300 },
+          // Leave ~28px of padding on the right so the percentile labels
+          // ("50th", "95th", ...) drawn past the last data point are not
+          // clipped by the canvas edge.
+          layout: { padding: { right: 30 } },
           scales: {
             x: {
               type: 'linear',