CI: bump deploy workflow's Node to 26, matching jsdom 30's engine requirement

jsdom 30 dropped support for Node 20 (needs ^22.22.2 || ^24.15.0 || >=26.0.0).
The CI workflow was still pinned to Node 20 from before that dependency bump,
so `npm test` failed in every run since — tests passed locally (on Node 26)
but the pipeline never got past the Test step, so neither of the last two
pushes actually deployed. The Dockerfile's own build stage doesn't need the
same bump: it only runs `npm run build` (tsc + vite), which doesn't touch
jsdom.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -11,7 +11,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: "20"
+          node-version: "26"
 
       - name: Install
         run: npm ci