Keep the installer centered when console geometry shifts

Remeasure the terminal at every screen redraw instead of once at boot:
the framebuffer can settle after the configurator starts (VM windows
especially), which left stale padding for the whole flow. Pad the
confirm-screen warnings with say so every line shares the same left
edge. Boot the test VM with SDL instead of GTK, whose GL display path
crops the guest framebuffer on HiDPI Wayland desktops.

Verified in a live VM by driving the whole flow over QMP and reading
each screen back from /dev/vcs1: all screens center on the real console
width.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/omarchy-iso-boot

@@ -161,7 +161,7 @@ qemu-system-x86_64 \
   -drive file="$ISO",media=cdrom,if=none,format=raw,id=cdrom0 \
   -device ide-cd,drive=cdrom0,bootindex=2 \
   -device virtio-vga-gl \
-  -display gtk,gl=on \
+  -display sdl,gl=on \
   -usb -device usb-tablet \
   "${NET_ARGS[@]}" \
   -boot menu=on \