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>

CHANGELOG.md

@@ -26,6 +26,14 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Changed
 
+- The version moved from the header to a footer: "Powered by Jarl v0.1.0",
+  linking to the release on Gitea. A build that isn't of a release tag
+  shows its commit instead, linked to the commit and marked `-modified`
+  with uncommitted changes. Before, every build of a commit after a
+  release still showed the release's version. `jarld version`,
+  `jarl --version`, `GET /status` and `/auth/whoami` report the same, the
+  latter two as `build` (docs/development.md).
+
 - The view in the zone list, and on the zone page, links to that view,
   and the views page highlights it. It used to be plain text, and the zone
   page linked to the views page as a whole.