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]>

config/plasma/xdg/plasmanotifyrc

@@ -0,0 +1,16 @@
+# Small cards, top right, out of the way of the bottom bar and of anything
+# a teacher is reading. Plasma's own notification service does this; the
+# classroom does not run a notification daemon of its own.
+[Notifications][$i]
+PopupPosition=TopRight
+PopupTimeout=6000
+LowPriorityPopups=false
+LowPriorityHistory=false
+
+[Jobs][$i]
+PermanentPopups=false
+InNotifications=true
+
+[DoNotDisturb][$i]
+WhenFullscreen=true
+WhenScreensMirrored=false