Check the machine before installing, and keep old shells' mise working

install.sh now stops early, with a clear message, if this isn't 64-bit
Arch, it's run as root, there's no internet, or sudo doesn't work.
system/preflight.sh then asks before going on with too little disk space
or / not on btrfs, and offers to switch another login screen to GDM and
PulseAudio to PipeWire. Keeping another login screen puts GDM in
services-off, so install and update stop failing to enable it; a
migration does the same on existing machines. It says nothing when all
is well, as refresh runs install.sh again.

Intel GPUs also get intel-media-driver for hardware video decoding, with
a migration for existing machines.

The mise migration links ~/.local/bin/mise to /usr/bin/mise instead of
deleting it, so shells opened before the update don't print an error at
every prompt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/1793457203.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+# Intel GPUs get their video decoding driver (intel-media-driver) too, so
+# video and calls don't decode on the CPU
+bash "$HOME/.local/share/coredesktop/system/vulkan.sh"