Drop the forced display mode, and let the kiosk user own its own cache

The screen was a badly seated cable - the enclosure was fouling the connector,
so the DDC and hotplug pins never mated while the video pairs partly did. That
is why it read 0 bytes of EDID on both ports and why forcing a mode "fixed" it.
With the cable seated properly HDMI-A-1 reports connected, hands over 256 bytes
of EDID and identifies itself as a DELL P2419H, so the force has nothing left to
do. Remove it and let EDID decide, which also means the mode is no longer
hardcoded to something that happened to match.

Separately, and the reason the screen then showed "Your Firefox profile cannot
be loaded": Xorg runs as root under Debian's wrapper with HOME pointing at the
kiosk user's, so the first mesa shader cache write created ~/.cache owned by
root and mode 0700. Firefox could not create ~/.cache/mozilla inside it and
never got as far as writing a profile - .mozilla/firefox held only Crash Reports
and Pending Pings, with no profiles.ini. Create ~/.cache and ~/.mozilla up front
owned by the user, so root only ever adds subdirectories to a directory the user
already owns.

Keeping the getty@tty1 enable and the userconfig.service mask: those were real
faults, not workarounds for this one.

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

host_vars/musicdolphin.yml

@@ -17,7 +17,3 @@ mediapi_install_kidsmusic: true
 mediapi_has_monitor: true
 musicmouse_version: deploy/musicdolphin
 
-# The attached screen reads back 0 bytes of EDID on both micro-HDMI ports and reports
-# "disconnected", so the kernel offers no modes and X dies with "no screens found".
-# Force the mode instead. Use the port nearest the USB-C jack (HDMI0 = HDMI-A-1).
-pi_kiosk_force_mode: "HDMI-A-1:1920x1080@60e"