build(deps): bump actions/setup-node from 4 to 6

Bumps [actions/setup-node](https://github.com/actions/setup-node) from 4 to 6.
- [Release notes](https://github.com/actions/setup-node/releases)
- [Commits](https://github.com/actions/setup-node/compare/v4...v6)

---
updated-dependencies:
- dependency-name: actions/setup-node
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/lint.yml

@@ -49,7 +49,7 @@ jobs:
         uses: actions/checkout@v7
 
       - name: Set up Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: "24"
           cache: npm

.github/workflows/tests.yml

@@ -151,7 +151,7 @@ jobs:
         run: cp app/config.sample.yaml app/config.yaml
 
       - name: Set up Node.js
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: "24"
           cache: npm

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:26-slim AS assets
+FROM node:25-slim AS assets
 
 WORKDIR /app
 COPY app/package.json ./