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/applications/ekklesia-dvd.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Name=DVD
+Comment=Play the disc in the drive
+Exec=/usr/local/bin/classroom-dvd
+Icon=media-optical-dvd
+Terminal=false
+Categories=AudioVideo;Player;
+StartupNotify=true
+StartupWMClass=vlc
+# Never in a menu or a launcher by itself. The DVD appears when a disc is
+# in the drive, through Plasma's own device notifier, and goes away with it.
+NoDisplay=true