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/kdeglobals

@@ -0,0 +1,60 @@
+# Ekklesia Classroom session policy.
+#
+# Installed to /etc/xdg/kdeglobals, where it is the bottom of Plasma's
+# configuration cascade. Every group is marked [$i] — immutable — so a
+# teacher, a stray click or a misbehaving application cannot override it
+# from the user's own configuration. This is KDE's own Kiosk mechanism:
+# nothing here is achieved by deleting a binary or hiding a menu entry.
+#
+# Lifting a restriction for maintenance is a one-line edit to this file
+# followed by a session restart, which is what makes the appliance
+# serviceable rather than merely locked.
+
+[KDE Action Restrictions][$i]
+# No way to reach a shell, a command line, or an arbitrary program.
+action/kdesktop_rmb=false
+action/kwin_rmb=false
+action/openwith=false
+action/switch_user=false
+action/start_new_session=false
+action/lock_screen=false
+run_command=false
+shell_access=false
+logout=false
+# The desktop and the panel are a fixed part of the appliance.
+editable_desktop_icons=false
+editable_bookmarks=false
+movable_toolbars=false
+plasma/plasmashell/unlockedDesktop=false
+plasma-desktop/scripting_console=false
+# Nothing in the classroom should offer to file a bug or open a manual.
+action/help_report_bug=false
+action/help_about_kde=false
+action/help_contents=false
+
+[General][$i]
+ColorScheme=EkklesiaDark
+AccentColor=79,179,168
+# The teacher's own words are not decoration: keep text rendering plain
+# and readable rather than styled.
+XftAntialias=true
+XftHintStyle=hintslight
+XftSubPixel=rgb
+
+[KDE][$i]
+widgetStyle=Breeze
+LookAndFeelPackage=org.ekklesia.classroom.desktop
+SingleClick=false
+AnimationDurationFactor=0.5
+
+[Icons][$i]
+Theme=breeze-dark
+
+[KFileDialog Settings][$i]
+Allow Expansion=false
+Automatically select filename extension=true
+Show Bookmarks=false
+Show Full Path=false
+Show Inline Previews=false
+Show Speedbar=false
+Sort directories first=true

config/plasma/xdg/krunnerrc

@@ -0,0 +1,9 @@
+# KRunner is a command line with a nicer face. The classroom has neither.
+[General][$i]
+Enabled=false
+ActivateWhenTypingOnDesktop=false
+[Plugins][$i]
+shellEnabled=false
+baloosearchEnabled=false
+krunner_appstreamEnabled=false
+krunner_shellEnabled=false

config/plasma/xdg/kscreenlockerrc

@@ -0,0 +1,9 @@
+# A classroom appliance must never present a password prompt to a teacher.
+# There is no lock screen, and nothing can bring one up: the action itself
+# is disabled in kdeglobals as well, so a keyboard shortcut cannot reach it.
+[Daemon][$i]
+Autolock=false
+LockOnResume=false
+LockOnStart=false
+LockGrace=0
+Timeout=0

config/plasma/xdg/ksmserverrc

@@ -0,0 +1,8 @@
+# The room is not somebody's account. Every session starts from the same
+# empty desktop, so what the last teacher left open is never the first
+# thing the next one sees.
+[General][$i]
+loginMode=emptySession
+confirmLogout=false
+offerShutdown=false
+shutdownType=0

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

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

config/plasma/xdg/plasmanotifyrc

@@ -0,0 +1,16 @@
+# Small cards, top right, out of the way of the bottom bar and of anything
+# a teacher is reading. Plasma's own notification service does this; the
+# classroom does not run a notification daemon of its own.
+[Notifications][$i]
+PopupPosition=TopRight
+PopupTimeout=6000
+LowPriorityPopups=false
+LowPriorityHistory=false
+
+[Jobs][$i]
+PermanentPopups=false
+InNotifications=true
+
+[DoNotDisturb][$i]
+WhenFullscreen=true
+WhenScreensMirrored=false