ci: bump actions to Node 24 runtimes to clear deprecation warning (#8)

GitHub is deprecating the Node 20 runtime for JS actions; every run printed a
warning that actions/checkout@v4, actions/setup-node@v4 and pnpm/action-setup@v4
were being force-run on Node 24. Bump each to its first Node 24 major (v5) across
all workflows — the smallest jump that clears the warning, avoiding the extra
behavior changes in checkout v6/v7 (credential persistence, fork-PR blocking)
that don't apply here. Our checkout jobs use push/pull_request, not
pull_request_target/workflow_run, so none are affected regardless.

Also bump the GitHub Pages actions in deploy-pages.yml (configure-pages v5->v6,
upload-pages-artifact v3->v5, deploy-pages v4->v5), which were likewise on Node
20. Inputs are unchanged; pnpm still pinned to 10.34.3 via the version input and
the packageManager field.

.github/workflows/docker-publish.yml

@@ -33,7 +33,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
 
       # The runtime image bakes the git commit for the AGPL-3.0 § 13 source
       # offer (config.commit → GET /api/instance/info). The .git dir is not in