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>

cmd/jarl/main.go

@@ -225,7 +225,7 @@ Exit codes: 0 ok, 1 error, 3 not authenticated or not allowed, 4 not found,
 5 conflict, 6 invalid or rejected by validation, 7 server or BIND error.`,
 		SilenceUsage:  true,
 		SilenceErrors: true,
-		Version:       version.Version,
+		Version:       version.String(),
 		PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
 			return checkFanOut(cmd)
 		},