ci: use PACKAGE_TOKEN for the registry login

GITEA_TOKEN is not set in these repos — the registry login sent a blank
password and Gitea's /v2/ endpoint returned unauthorized. Switch the
image-build login, the app template, and the README to PACKAGE_TOKEN, the
PAT the other repos already use (Syrinx pairs gitea.actor + PACKAGE_TOKEN
successfully). The PAT needs write:package here and read:package in app
repos.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/build.yml

@@ -29,7 +29,7 @@ jobs:
       image: git.helu.ca/r/android:2026.06
       credentials:
         username: ${{ gitea.actor }}
-        password: ${{ secrets.GITEA_TOKEN }}
+        password: ${{ secrets.PACKAGE_TOKEN }}
     steps:
       - name: Checkout
         uses: actions/checkout@v4