feat: run prettier after npm updates so format bumps stay green

A prettier version bump changes prettier's canonical output, so files
that were correctly formatted under the old version now fail
`prettier --check` in CI — the update PR goes red and never automerges.
This happened on castaway with prettier 3.8.3 -> 3.9.6 (two files
reformatted: a union type collapsed to one line, a Svelte closing tag
pulled onto the attribute line).

Add a npm-scoped postUpgradeTasks that runs `npm run format` once per
branch after the upgrade and commits the result. `--if-present` makes it
a no-op in npm repos without a format script; matchManagers npm keeps it
away from the gradle/maven and firmware repos that share this preset.

Requires the self-hosted Renovate instance to whitelist the command via
allowedCommands (e.g. RENOVATE_ALLOWED_COMMANDS='^npm run format') — that
is a global/self-hosted-only option and cannot be set from this preset.

Signed-off-by: Peter Siegmund <peter.siegmund@entwicklung.eq-3.de>

default.json

@@ -18,6 +18,15 @@
     "automerge": true
   },
   "packageRules": [
+    {
+      "description": "Nach npm-Updates Prettier laufen lassen: Formatierungs-Bumps (z.B. prettier selbst) formatieren bestehende Dateien um, sonst schlaegt prettier --check in der CI fehl. --if-present no-op't in Repos ohne format-Script. Benoetigt allowedCommands in der self-hosted Renovate-Config.",
+      "matchManagers": ["npm"],
+      "postUpgradeTasks": {
+        "commands": ["npm run format --if-present"],
+        "fileFilters": ["**/*"],
+        "executionMode": "branch"
+      }
+    },
     {
       "description": "npm devDependencies (non-major) gesammelt, automerge bei gruener CI",
       "matchManagers": ["npm"],