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/kwinrulesrc

@@ -0,0 +1,48 @@
+# Window rules.
+#
+# There are three, they are all about size, and none of them is about
+# stacking. Dialogs staying in front of the window that opened them,
+# secondary windows floating over the main one instead of resizing it, and
+# a print dialog being the window in front are all ordinary window-manager
+# behaviour here, so there is nothing to write down.
+#
+# `types=1` is NET::Normal only. Without it a rule that maximises would
+# also maximise the dialogs, which is exactly the behaviour the classroom
+# is moving away from.
+
+[General]
+count=3
+rules=classroom-browser,classroom-wonderink,classroom-video
+
+[classroom-browser]
+Description=The classroom browser fills the screen
+wmclass=google-chrome
+wmclassmatch=1
+wmclasscomplete=false
+types=1
+maximizehoriz=true
+maximizehorizrule=3
+maximizevert=true
+maximizevertrule=3
+
+[classroom-wonderink]
+Description=Wonder Ink fills the screen
+# Chrome derives this from the --app URL. Measured on a running session
+# rather than guessed; it is also what the launcher's StartupWMClass says,
+# and the two have to agree or the panel shows two buttons for one app.
+wmclass=chrome-app.wonderink.org__-Default
+wmclassmatch=1
+wmclasscomplete=false
+types=1
+maximizehoriz=true
+maximizehorizrule=3
+maximizevert=true
+maximizevertrule=3
+
+[classroom-video]
+Description=Video is an appliance screen, not a window
+wmclass=vlc
+wmclassmatch=2
+wmclasscomplete=false
+noborder=true
+noborderrule=2