Install Yarn explicitly in the deploy workflow

setup-node provides Node but not Yarn; the workflow was relying on the runner
image shipping it, as GitHub's hosted images do. When the act_runner image
changed, `yarn install` began failing with exit 127 and every push since
6964ce7 (2026-07-19) silently stopped deploying — a dead pipeline reads the
same as a site that did not need to change.

Pinned to 1.x: the lockfile is v1 and `--frozen-lockfile` is v1 syntax.

.gitea/workflows/deploy.yaml

@@ -14,6 +14,15 @@ jobs:
         with:
           node-version: "22"
 
+      # setup-node installs Node but not Yarn. GitHub's hosted images happen to
+      # ship Yarn preinstalled, so this workflow was silently depending on the
+      # image rather than on anything declared here — and broke the moment the
+      # act_runner image changed, with `yarn: command not found` (exit 127).
+      # Pin to 1.x: the lockfile is v1 format and `--frozen-lockfile` is v1
+      # syntax (v2+ renamed it `--immutable`).
+      - name: Install Yarn
+        run: npm install -g yarn@1
+
       - name: Install dependencies
         run: yarn install --frozen-lockfile