feat(iso): remove splash.png, show EASY-BEE ASCII art in GRUB text mode

The graphical splash had "BEE / HARDWARE AUDIT" baked into the PNG,
overriding the echo ASCII art. Replace with a plain black background
so the EASY-BEE block-char banner from grub.cfg echo commands is visible.

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

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

@@ -1,4 +1,4 @@
-desktop-image: "../splash.png"
+desktop-color: "#000000"
 title-color: "#f5a800"
 title-font: "Unifont Regular 16"
 title-text: ""