feat: support user preview configuration and document UI

scripts/test-cli.sh

@@ -85,6 +85,14 @@ shown="$workdir/show-config"
 assert_contains "tty = 2" "$shown"
 assert_contains "pam_service" "$shown"
 
+user_config_home="$workdir/config-home"
+mkdir -p "$user_config_home/latchd"
+sed 's/^tty = 2$/tty = 7/' "$config" >"$user_config_home/latchd/config.toml"
+user_shown="$workdir/user-show-config"
+HOME="$workdir/home" XDG_CONFIG_HOME="$user_config_home" \
+    "$binary" --preview --show-config --no-log >"$user_shown"
+assert_contains "tty = 7" "$user_shown"
+
 unknown="$workdir/unknown.toml"
 printf 'unknown = true\n' >"$unknown"
 if "$binary" --preview --no-tui --no-log --config "$unknown" >"$workdir/unknown.out" 2>&1; then