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-wonderink.desktop

@@ -0,0 +1,15 @@
+[Desktop Entry]
+Type=Application
+Name=Wonder Ink
+GenericName=Lesson
+Comment=The Wonder Ink lesson for this room
+Exec=/usr/local/bin/classroom-wonderink
+Icon=ekklesia-wonderink
+Terminal=false
+Categories=Education;
+StartupNotify=true
+# How the panel knows that the window on screen belongs to this launcher.
+# Chrome derives an app id from the --app URL, and this must match it
+# exactly or the launcher and the window become two separate buttons.
+StartupWMClass=chrome-app.wonderink.org__-Default
+X-GNOME-SingleWindow=true