fix(grub): fix bitmap error and menu rendering

- Convert bee-logo.png to RGBA (color type 6) and strip all metadata
  chunks (cHRM, bKGD, tIME, tEXt) that confuse GRUB's minimal PNG parser
- Move terminal_output gfxterm before insmod png / theme load so the
  theme initialises in an active gfxterm context
- Remove echo ASCII art banner from grub.cfg — with gfxterm active and
  no terminal_box in the theme, echo output renders over the menu area
- Fix icon_heigh typo → icon_height; increase item_height 16→20 with
  item_padding 0→2 for reliable text rendering in boot_menu

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

iso/builder/config/bootloaders/grub-efi/config.cfg

@@ -23,9 +23,9 @@ insmod serial
 serial --unit=0 --speed=115200 --word=8 --parity=no --stop=1
 
 insmod gfxterm
-insmod png
-
-source /boot/grub/theme.cfg
 
 terminal_input console serial
 terminal_output gfxterm serial
+
+insmod png
+source /boot/grub/theme.cfg