[monitoring] uk-payslip Panel 13: stacked bars + sum-in-legend

"Monthly cash flow — tax impact (RSU excluded)" was already stacking
group A in normal mode but rendered as 70%-opacity filled lines — the
overlap made the total-per-month figure visually inaccessible.

Switch drawStyle to bars (100% fill, 0-width lineWidth, no per-point
markers) so each month reads as a single stacked bar whose top edge is
the total cash-side deduction. Add "sum" to legend.calcs so the
tax-year totals per series show in the legend table alongside last and
max.

Panel 11 (Tax & pension — monthly, RSU-inclusive) retains the line/
area style so the two panels remain visually distinct.

stacks/monitoring/modules/monitoring/dashboards/uk-payslip.json

@@ -921,20 +921,20 @@
           "unit": "currencyGBP",
           "custom": {
             "axisPlacement": "auto",
-            "drawStyle": "line",
+            "drawStyle": "bars",
-            "fillOpacity": 70,
+            "fillOpacity": 100,
             "gradientMode": "none",
             "hideFrom": {
               "legend": false,
               "tooltip": false,
               "viz": false
             },
-            "lineWidth": 1,
+            "lineWidth": 0,
             "pointSize": 4,
             "scaleDistribution": {
               "type": "linear"
             },
-            "showPoints": "auto",
+            "showPoints": "never",
             "spanNulls": false,
             "stacking": {
               "group": "A",
@@ -1028,7 +1028,8 @@
         "legend": {
           "calcs": [
             "last",
-            "max"
+            "max",
+            "sum"
           ],
           "displayMode": "table",
           "placement": "bottom"