fix(dashboardAPI): MGC dashboard — drop dead Scaling panel, show group Mode/RelDistPeak

- Remove the "Scaling" stat panel: it queried field 'scaling' that machineGroup
  never emits, so it always rendered "No data".
- The "Mode" and "Rel Dist Peak" panels were stripped by the #39 no-duplication
  rule because child pumps emit fields of the same name ('mode', 'relDistFromPeak').
  But those are the GROUP's own measurement, never a true duplicate of per-pump
  series. Mark them emittedFields:[] (the existing "always keep" convention used by
  the injected pump panels) so the group-level status/metric renders.

Verified live: MGC dashboard now shows Mode "optimalControl", Abs/Rel Dist Peak.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config/machineGroup.json

@@ -76,58 +76,7 @@
       "title": "Mode",
       "type": "stat",
       "meta": {
-        "emittedFields": [
-          "mode"
-        ]
-      }
-    },
-    {
-      "datasource": {
-        "type": "influxdb",
-        "uid": "cdzg44tv250jkd"
-      },
-      "fieldConfig": {
-        "defaults": {
-          "thresholds": {
-            "mode": "absolute",
-            "steps": [
-              {
-                "color": "blue",
-                "value": null
-              }
-            ]
-          }
-        },
-        "overrides": []
-      },
-      "gridPos": {
-        "h": 4,
-        "w": 6,
-        "x": 6,
-        "y": 1
-      },
-      "id": 3,
-      "options": {
-        "reduceOptions": {
-          "calcs": [
-            "lastNotNull"
-          ]
-        },
-        "colorMode": "value",
-        "graphMode": "none"
-      },
-      "targets": [
-        {
-          "query": "from(bucket: \"${bucket}\")\n  |> range(start: -7d)\n  |> filter(fn:(r) => r._measurement==\"${measurement}\" and r._field==\"scaling\")\n  |> group(columns:[\"_field\"])\n  |> last()",
-          "refId": "A"
-        }
-      ],
-      "title": "Scaling",
-      "type": "stat",
-      "meta": {
-        "emittedFields": [
-          "scaling"
-        ]
+        "emittedFields": []
       }
     },
     {
@@ -240,9 +189,7 @@
       "title": "Rel Dist Peak",
       "type": "stat",
       "meta": {
-        "emittedFields": [
-          "relDistFromPeak"
-        ]
+        "emittedFields": []
       }
     },
     {