Fix kernel panic by adding ramdisk_size to iPXE kernel params

The Proxmox initrd is too large for the default ramdisk allocation,
causing VFS to fail mounting root. Add ramdisk_size=16777216 (16GB)
along with rw, quiet, and splash=verbose for proper installer boot.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/pxe/ipxe.go

@@ -12,7 +12,7 @@ func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}
-kernel %s vga=791 video=vesafb:lfb:on
+kernel %s ramdisk_size=16777216 vga=791 video=vesafb:lfb:on rw quiet splash=verbose
 initrd %s
 boot
 `, img.Name, kernelURL, initrdURL)