monitoring(wealth): fix x-axis label formatting on yearly bars

The default fieldConfig unit (percent on Yearly investment return %,
currencyGBP on Annual change decomposition) was being applied to the
"year" string column too — so x-axis labels rendered as "2024%" and
"£2,024" respectively. Add field overrides on the "year" column to
force unit=string. The earlier "tax_year" panels weren't affected
because "2024/25" doesn't parse as a number; "2024" did.

stacks/monitoring/modules/monitoring/dashboards/wealth.json

@@ -507,7 +507,14 @@
             "lineWidth": 1
           }
         },
-        "overrides": []
+        "overrides": [
+          {
+            "matcher": {"id": "byName", "options": "year"},
+            "properties": [
+              {"id": "unit", "value": "string"}
+            ]
+          }
+        ]
       },
       "options": {
         "barRadius": 0,
@@ -553,6 +560,12 @@
           }
         },
         "overrides": [
+          {
+            "matcher": {"id": "byName", "options": "year"},
+            "properties": [
+              {"id": "unit", "value": "string"}
+            ]
+          },
           {
             "matcher": {"id": "byName", "options": "contributions"},
             "properties": [