ci: remove npm cache from setup-node to speed up lint-and-test job

The Forgejo runner uses catthehacker/ubuntu:act-latest which does not have
a real GitHub Actions cache backend. actions/setup-node@v4 with cache: npm
spends ~4m44s trying to restore a non-existent cache during setup, and then
~4m40s in the post-job hook trying to save the cache during 'Complete job'.

- Remove cache: npm and cache-dependency-path from setup-node step
- npm ci without cache is fast enough for this project size (~10-20s)

Expected result: lint-and-test job drops from ~11m to ~2m total.

.forgejo/workflows/ci.yml

@@ -21,8 +21,6 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 24
-          cache: npm
-          cache-dependency-path: frontend/package-lock.json
 
       - uses: https://github.com/actions/setup-java@v4
         with: