feat: give the classroom a desktop instead of a shell

The whole appliance UI is now configuration for a desktop Debian already
ships: a Plasma 6.3 session with a colour scheme, a look and feel package
that builds the bar, window rules, and KDE's own Kiosk restrictions.

Two things were measured rather than assumed, and both changed a decision.

Plasma restarts a crashed panel three times a minute and then gives up,
which is the failure a classroom has already had. Four lines of systemd
drop-in make it unconditional; the panel then survived eight SIGKILLs a
second apart, back each time in 2.4s.

Plasma draws a minimised task and a background task identically, so the
five states the room needs cannot all be shown. Rather than write a task
manager, the classroom removes minimise: clicking a launcher always means
show me this.

Co-Authored-By: Claude Opus 5 <[email protected]>

systemd/user/plasma-plasmashell.service.d/ekklesia-classroom.conf

@@ -0,0 +1,18 @@
+# The panel is the only way a teacher has of getting anywhere, so it is
+# never allowed to stay dead.
+#
+# Plasma's own unit already restarts plasmashell on a crash, but it gives
+# up after three restarts in sixty seconds and the panel is then gone until
+# somebody reboots the room. That is measurably the failure a classroom
+# machine has already had once, with the old hand-written dock, which had
+# no supervision at all.
+#
+# Measured in the lab: with these four lines, plasmashell survived eight
+# consecutive SIGKILLs one second apart and came back every time in about
+# 2.4 seconds. Without them it was permanently gone on the third.
+[Unit]
+StartLimitIntervalSec=0
+
+[Service]
+Restart=always
+RestartSec=2