Center the installer screens and move the effects to ttfx

The progress screen and the finish screen now center vertically the way the
greeter does, instead of stacking from row one. Both derive their block height
from the logo, so the frame the installer sits on for several minutes is
composed rather than top-aligned. The progress screen redraws itself if the
console resizes, which the framebuffer still does late in VMs.

Centering the finish screen turned up a spacing bug: gum confirm paints
prompt/blank/button from the cursor and clears all three rows, so anchoring one
row below the title left three blank rows under it, not one. Anchoring a row
higher closes that to the two rows gum allows, one more than the greeter's frame.

Swap python-terminaltexteffects for ttfx. The flags carry over, but two things
do not. ttfx resolves even indexed stops to truecolor, so the greeter needs
--xterm-colors to keep its palette indexed: the console reduces 256-colour codes
well and 24-bit ones badly, and the settle colour has to match the logo drawn
underneath with gum style --foreground 2. And ttfx centres its text two columns
right of plain centering, so both screens hand it a canvas two columns narrower
than the console or the logo jumps sideways when the effect starts.

The finish screen uses smoke rather than laseretch, at 60fps rather than 260 —
smoke finishes in under a second at the rate laseretch was tuned for, which
reads as a flicker instead of a send-off. Smoke also survives the console's
16-colour collapse better, carrying its motion in glyphs rather than gradients.

ttfx is in the ISO's package list and timeout is coreutils, so neither needs an
availability check.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

builder/build-iso.sh

@@ -117,7 +117,7 @@ cp "/tmp/$NODE_FILENAME" "$build_cache_dir/airootfs/opt/packages/"
 # The selected omarchy-settings package is needed here so its post_install hook
 # drops Omarchy's plymouthd.conf into /etc/plymouth before mkarchiso builds the
 # live initramfs.
-arch_packages=(linux-t2 git gum jq openssl plymouth python-terminaltexteffects tzupdate omarchy-keyring "$OMARCHY_SETTINGS_PACKAGE" lvm2 cryptsetup parted)
+arch_packages=(linux-t2 git gum jq openssl plymouth ttfx tzupdate omarchy-keyring "$OMARCHY_SETTINGS_PACKAGE" lvm2 cryptsetup parted)
 printf '%s\n' "${arch_packages[@]}" >> "$build_cache_dir/packages.x86_64"
 
 # The live ISO boots linux-t2 (see airootfs/etc/mkinitcpio.d/linux-t2.preset), so