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/solid/ekklesia-dvd.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Service
+# Solid decides when this exists. Plasma's device notifier offers it while
+# a video DVD is in the drive and withdraws it when the disc is ejected, so
+# a room without a drive shows nothing and a room with one needs no setting
+# changed. Nothing polls.
+X-KDE-Solid-Predicate=[[ StorageVolume.ignored == false AND StorageVolume.usage == 'Other' ] AND [ OpticalDisc.availableContent  & 'VideoDvd' ]]
+Actions=Play;
+
+[Desktop Action Play]
+Name=Play the DVD
+Icon=media-optical-dvd
+Exec=/usr/local/bin/classroom-dvd %f