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/lookandfeel/contents/defaults

@@ -0,0 +1,31 @@
+# Everything the classroom's appearance is made of, in one place.
+#
+# Plasma reads this when the look and feel is applied, so the panel, the
+# window decorations, the notifications and every Qt dialog in the session
+# are described here rather than in a stylesheet per surface.
+
+[kdeglobals][KDE]
+widgetStyle=Breeze
+
+[kdeglobals][General]
+ColorScheme=EkklesiaDark
+AccentColor=79,179,168
+
+[kdeglobals][Icons]
+Theme=breeze-dark
+
+[plasmarc][Theme]
+name=default
+
+[kcminputrc][Mouse]
+cursorTheme=breeze_cursors
+
+[kwinrc][org.kde.kdecoration2]
+library=org.kde.breeze
+theme=Breeze
+
+[Wallpaper]
+Image=/usr/share/backgrounds/ekklesia/classroom-calm.png
+
+[KSplash]
+Theme=None