Switch to sanboot for PXE ISO delivery

Proxmox has no kernel parameter for HTTP ISO fetch. Instead, use
iPXE's sanboot command to present the ISO as a virtual CD over HTTP.
The installer finds it as a block device and boots normally.

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

internal/pxe/ipxe.go

@@ -7,17 +7,11 @@ import (
 )
 
 func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
-	kernelURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.KernelPath)
-	initrdURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.InitrdPath)
 	isoURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.ISOPath)
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}
-echo Loading kernel...
-kernel %s ramdisk_size=16777216 vga=791 video=vesafb:lfb:on rw quiet splash=verbose proxmox-iso-url=%s initrd=initrd
-echo Loading initrd...
-initrd --name initrd %s
-echo Booting...
-boot
-`, img.Name, kernelURL, isoURL, initrdURL)
+echo Booting ISO via HTTP SAN...
+sanboot %s
+`, img.Name, isoURL)
 }