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/kwinrc

@@ -0,0 +1,102 @@
+# The classroom's window manager.
+#
+# The shape of this file is the argument for the whole architecture: window
+# stacking, minimising, dialog placement and full-screen media are settings
+# here, not code somewhere in the classroom.
+
+[Compositing][$i]
+Enabled=true
+# Let KWin choose between OpenGL and software rendering by itself. A
+# classroom machine that loses acceleration after a kernel update should
+# come up slower, not blank.
+WindowsBlockCompositing=true
+AllowTearing=false
+LatencyPolicy=Medium
+
+[Desktops][$i]
+Number=1
+Rows=1
+# One desktop, so a teacher cannot land on an empty one and conclude the
+# computer has lost their work.
+
+[Windows][$i]
+Placement=Centered
+FocusPolicy=ClickToFocus
+AutoRaise=false
+DelayFocus=0
+FocusStealingPreventionLevel=1
+# A dialog belongs to the window that opened it and stays with it. This is
+# what keeps a print dialog in front of the page it is printing without
+# anything in the classroom arranging it.
+SeparateScreenFocus=false
+ActiveMouseScreen=false
+TitlebarDoubleClickCommand=Maximize
+MaximizeButtonLeftClickCommand=Maximize
+MaximizeButtonMiddleClickCommand=Maximize
+MaximizeButtonRightClickCommand=Maximize
+
+[MouseBindings][$i]
+CommandAllKey=Meta
+CommandAll1=Move
+CommandAll2=Nothing
+CommandAll3=Nothing
+CommandTitlebarWheel=Nothing
+CommandAllWheel=Nothing
+
+[TabBox][$i]
+# No window switcher overlay. The bottom bar is the way between windows.
+ShowTabBox=false
+
+[Plugins][$i]
+# Effects the classroom uses, and nothing else. Every line that is false is
+# an effect that will not be loaded at all.
+blurEnabled=true
+contrastEnabled=true
+kwin4_effect_fadeEnabled=true
+kwin4_effect_translucencyEnabled=false
+kwin4_effect_dialogparentEnabled=true
+kwin4_effect_squashEnabled=true
+kwin4_effect_maximizeEnabled=true
+kwin4_effect_scaleEnabled=true
+kwin4_effect_fullscreenEnabled=true
+slidingpopupsEnabled=true
+screenedgeEnabled=false
+cubeEnabled=false
+desktopgridEnabled=false
+overviewEnabled=false
+windowviewEnabled=false
+wobblywindowsEnabled=false
+magiclampEnabled=false
+slideEnabled=false
+zoomEnabled=false
+mouseclickEnabled=false
+showfpsEnabled=false
+thumbnailasideEnabled=false
+kwin4_effect_dimscreenEnabled=false
+kwin4_effect_morphingpopupsEnabled=false
+
+[Effect-blur][$i]
+BlurStrength=8
+NoiseStrength=2
+
+[org.kde.kdecoration2][$i]
+library=org.kde.breeze
+theme=Breeze
+# A pin and a close button, and nothing else.
+#
+# The pin is the Live View story: keeping a window above the others is a
+# window-manager state, not a feature the classroom has to invent. There is
+# deliberately no minimise button, because a minimised window is the one
+# thing a teacher cannot see the state of — measured, not assumed. Double
+# clicking the title bar still maximises, so nothing is unreachable.
+ButtonsOnLeft=
+ButtonsOnRight=FX
+ShowToolTips=false
+BorderSize=None
+BorderSizeAuto=false
+
+[Script-desktopchangeosd][$i]
+Enabled=false
+
+[Xwayland][$i]
+Scale=1