scripts: optically center the single GRUB entry under the boot card

The canary boot showed the entry sitting noticeably left of the centered logo.
gfxmenu's boot_menu draws items left-aligned inside the box with a ~29px inset
and offers no item alignment, so centering means placing the box: left 25%->40%,
width 50%->35%. Measured off the canary screenshot, not guessed; the comment
records the measurement so a future label change gets re-measured instead of
re-guessed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nn3VgQk9iwEGgyx6QJ2NvE

scripts/iso/grub/felhom-theme.txt

@@ -15,11 +15,17 @@ title-text: ""
 desktop-image: "background.png"
 desktop-color: "#0D1117"
 
-# The single entry ("Felhom telepítés"). Below the card, horizontally centered.
+# The single entry ("Felhom telepítés"). Below the card, optically centered under it.
+#
+# boot_menu has no item alignment — items are drawn left-aligned inside the box, at a ~29px inset
+# (measured off the canary boot: box left 25% = 256px put the glyphs at x≈285). So centering is done
+# by placing the BOX, not the text: 'Felhom telepítés' renders ~133px wide, so left=40% (=410px)
+# puts the text at ~439..572, centered on ~505 against the card's 512. If the entry label ever
+# changes length, re-measure from a canary screenshot rather than assuming.
 + boot_menu {
-  left = 25%
+  left = 40%
   top = 70%
-  width = 50%
+  width = 35%
   height = 14%
 
   item_color = "#E6EDF3"