feat(spaceshd): configurable default shell

Add a config.rs module that loads optional ~/.spacesh/config.toml and a
default_shell() resolver: SPACESH_SHELL env -> config.toml default_shell ->
login shell from the passwd DB -> $SHELL -> /bin/sh. The passwd lookup
matters under launchd where $SHELL is absent. All plain-panel spawn sites in
server.rs now use the resolver. Adds toml + libc deps.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -26,5 +26,7 @@ portable-pty = "0.8"
 alacritty_terminal = "0.25"
 fs2 = "0.4"
 dirs = "5"
+toml = "0.8"
+libc = "0.2"
 clap = { version = "4", features = ["derive"] }
 clap_complete = "4"