feat: add idle timeout and parent process death detection

Prevents orphaned bbs-md processes when SSH sessions disconnect
uncleanly. Adds two mechanisms:

- Parent death check: polls getppid() each loop iteration and exits
  when the parent (sshd) dies, detecting session orphaning immediately.
- Idle timeout: configurable idle_timeout_minutes in bbs.toml (default
  0 = disabled) exits after no keyboard/mouse input for the duration.

Cargo.toml

@@ -18,3 +18,4 @@ walkdir = "2.5"
 ureq = "2.12"
 ratatui-image = { version = "10.0", default-features = false }
 image = "0.25"
+libc = "0.2"