Add app-store release automation (Makefile appstore target + publish workflow)

[Martin-1997]

Add app-store release automation (Makefile appstore target + publish workflow)

Problem

The App Store requires a release archive with a single top-level folder named exactly the app id (timetracker/, containing appinfo/info.xml). GitHub's auto-generated "Source code" tarball names the top folder timetracker-<version>/, which the store rejects. We should build a correctly-structured, signed tarball and publish it automatically on each GitHub release, so new versions no longer have to be packaged and uploaded by hand. More information is available here: https://nextcloudappstore.readthedocs.io/en/latest/developer.html

Tasks

• Add a make appstore target that produces build/artifacts/timetracker.tar.gz:
  • run the JS build (cd js && npm ci && npm run build) so compiled assets are included;
  • rsync the runtime files into a timetracker/ folder, excluding dev cruft (.git, js/node_modules, js/src if only built output ships, tests, Makefile, *.md, build/CI configs, etc.);
  • tar -czf the timetracker/ folder so it is the single top-level directory.
• Add a publish workflow .github/workflows/appstore-build-publish.yml, triggered on release: published, based on the official template in [nextcloud/.github](https://github.com/nextcloud/.github/blob/master/workflow-templates/appstore-build-publish.yml). It should: checkout the tag → make appstore → sign → attach the tarball to the GitHub release → push to the store via nextcloud-releases/nextcloud-appstore-push-action.
• Configure the repository secrets the workflow expects (per the template's documentation).
• Confirm pre-releases upload as nightly versions (the action/template handles this).

Definition of Done

• Cutting a GitHub release automatically publishes the new version to apps.nextcloud.com within a few minutes.
• The uploaded tarball has timetracker/ as its single top-level folder and passes the store's signature check.
• The release/secret setup is documented in the repo README or CONTRIBUTING.

Test

• Dry-run make appstore locally and verify tar tzf build/artifacts/timetracker.tar.gz | head shows everything under timetracker/.
• Cut a test (pre-)release and confirm it appears as a nightly on the store.

Notes

• Manual fallback if needed: the [krankerl](https://github.com/ChristophWurst/krankerl) CLI (package + publish) or the store's web upload form / REST API with an openssl dgst -sha512 … | openssl base64 signature.