Lets `backlog show` work without arguments when the env var is set,
consistent with the task subcommands.

Assisted-by: Claude Opus 4.6 via pi

diff --git a/src/bin/ranger/commands/backlog.rs b/src/bin/ranger/commands/backlog.rs
index 3d05216..8c23b48 100644
--- a/src/bin/ranger/commands/backlog.rs
+++ b/src/bin/ranger/commands/backlog.rs
@@ -18,6 +18,7 @@ pub enum BacklogCommands {
     /// Show a backlog's details
     Show {
         /// Backlog name
+        #[arg(env = "RANGER_DEFAULT_BACKLOG")]
         name: String,
     },
 }