fix: add GRUB timeout for auto-boot

GRUB had no timeout set, causing it to wait indefinitely at the boot
menu. Added 5-second timeout so the default entry (live system) boots
automatically. Required for automated/serial console testing.

💘 Generated with Crush

Assisted-by: GLM-5.1 via Crush <crush@charm.land>

config/bootloaders/grub-pc/config.cfg

@@ -1,4 +1,5 @@
 set default=0
+set timeout=5
 
 # Serial console for demo/validation mode
 serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1