Show the build in a footer, linked to its release or commit

The version moves from the header to a "Powered by Jarl" footer. A build
of a release tag (HEAD tagged v<VERSION>, no tracked changes; the release
workflow sets RELEASE_TAG) shows and links the release; any other build
shows and links its commit, marked -modified with uncommitted changes.
jarld version, jarl --version, /status and /auth/whoami report the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -48,6 +48,10 @@ jobs:
         run: make test
 
       - name: Build
+        env:
+          # Checked against VERSION above; don't depend on the checkout
+          # having the tag.
+          RELEASE_TAG: ${{ github.ref_name }}
         run: |
           export DEB_MAINTAINER="$(git log -1 --format='%an <%ae>')"
           make dist