Fix GRUB bootloader config dir: grub-pc → grub-efi

Build uses --bootloaders "grub-efi,syslinux" so live-build reads
config/bootloaders/grub-efi/ for the UEFI GRUB config. The directory
was incorrectly named grub-pc, causing live-build to ignore our custom
grub.cfg and generate a default one (missing toram, GSP-off entries).

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

iso/builder/config/bootloaders/grub-pc/live-theme/theme.txt

@@ -1,51 +0,0 @@
-desktop-color: "#000000"
-title-color: "#f5a800"
-title-font: "Unifont Regular 16"
-title-text: ""
-message-font: "Unifont Regular 16"
-terminal-font: "Unifont Regular 16"
-
-#help bar at the bottom
-+ label {
-        top = 100%-50
-        left = 0
-        width = 100%
-        height = 20
-        text = "@KEYMAP_SHORT@"
-        align = "center"
-        color = "#5a4800"
-        font = "Unifont Regular 16"
-}
-
-#boot menu
-+ boot_menu {
-        left = 20%
-        width = 60%
-        top = 62%
-        height = 38%-80
-        item_color = "#c88000"
-        item_font = "Unifont Regular 16"
-        selected_item_color= "#f5a800"
-        selected_item_font = "Unifont Regular 16"
-        item_height = 16
-        item_padding = 0
-        item_spacing = 4
-        icon_width = 0
-        icon_heigh = 0
-        item_icon_space = 0
-}
-
-#progress bar
-+ progress_bar {
-        id = "__timeout__"
-        left = 20%
-        top = 100%-80
-        height = 14
-        width = 60%
-        font = "Unifont Regular 16"
-        text_color = "#0a0a00"
-        fg_color = "#f5a800"
-        bg_color = "#2a2200"
-        border_color = "#5a4800"
-        text = "@TIMEOUT_NOTIFICATION_LONG@"
-}