chore(deps): group node and pnpm updates across mise and version files

Add packageRules to keep mise.toml in sync with the files it mirrors
when Renovate raises version-bump PRs:

- node: groups mise.toml and frontend/.nvmrc (nvm manager) into one PR
- pnpm: groups mise.toml and frontend/package.json#packageManager
  (npm manager) into one PR

Without these rules Renovate would open separate PRs for each file,
allowing them to drift out of sync.

renovate.json

@@ -75,6 +75,26 @@
       "extends": [
         "schedule:weekly"
       ]
+    },
+    {
+      "groupName": "node",
+      "matchPackageNames": [
+        "node"
+      ],
+      "matchManagers": [
+        "mise",
+        "nvm"
+      ]
+    },
+    {
+      "groupName": "pnpm",
+      "matchPackageNames": [
+        "pnpm"
+      ],
+      "matchManagers": [
+        "mise",
+        "npm"
+      ]
     }
   ]
 }