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/chrome/policies/recommended/ekklesia-classroom.json

@@ -1,6 +1,6 @@
 {
   "RestoreOnStartup": 4,
   "RestoreOnStartupURLs": [
-    "https://app.wonderink.org/"
+    "https://www.youtube.com/"
   ]
 }