fix(live-usb): Add emergency shell boot entry and enable systemd status

- Add Emergency Shell boot option for debugging
- Enable ShowStatus=yes to see boot progress
- Set 30s timeout for services to prevent infinite hangs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

image/build-live-usb.sh

@@ -206,11 +206,13 @@ chroot "${ROOTFS}" systemctl set-default multi-user.target 2>/dev/null || true
 mkdir -p "${ROOTFS}/etc/live/config.conf.d"
 echo 'LIVE_CONFIG_NOAUTOLOGIN=true' > "${ROOTFS}/etc/live/config.conf.d/no-autologin.conf"
 
-# Quiet boot
+# Boot status (show for debugging)
 mkdir -p "${ROOTFS}/etc/systemd/system.conf.d"
-cat > "${ROOTFS}/etc/systemd/system.conf.d/quiet-boot.conf" <<EOF
+cat > "${ROOTFS}/etc/systemd/system.conf.d/boot.conf" <<EOF
 [Manager]
-ShowStatus=no
+ShowStatus=yes
+DefaultTimeoutStartSec=30s
+DefaultTimeoutStopSec=30s
 EOF
 
 # Disable console spam
@@ -980,6 +982,11 @@ menuentry "SecuBox Live (Auto-Check HW)" {
     linux ($live)/live/vmlinuz boot=live live-media-path=live components nomodeset console=tty0 secubox.hwcheck=1
     initrd ($live)/live/initrd.img
 }
+
+menuentry "SecuBox Live (Emergency Shell)" {
+    linux ($live)/live/vmlinuz boot=live live-media-path=live components nomodeset console=tty0 systemd.unit=emergency.target
+    initrd ($live)/live/initrd.img
+}
 GRUBCFG
 
 cp "${MNT}/esp/boot/grub/grub.cfg" "${MNT}/esp/EFI/BOOT/grub.cfg"