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/jarld/admin.go

@@ -383,7 +383,7 @@ func versionCmd() *cobra.Command {
 		Use:   "version",
 		Short: "Print the version",
 		Run: func(cmd *cobra.Command, args []string) {
-			fmt.Println("jarld", version.Version)
+			fmt.Println("jarld", version.String())
 		},
 	}
 }