build: make the classroom desktop something a .deb installs

Building the package found two things reading the tree did not.

dpkg refused it outright: plasma-workspace already owns
/etc/xdg/plasmanotifyrc and friends, and two packages cannot own one
path. The room's settings move to a directory of their own with
/etc/environment.d putting it in front of /etc/xdg, which is KConfig's
own cascade rather than a file fight.

The postinst then failed on a chmod for a command that no longer
exists. It now walks what the package installed instead of naming
files, removes the login shell that used to start sway on tty1, and
enables SDDM and the audio service.

Verified by installing the built package on a fresh machine and
starting the session from it: restrictions in force, palette applied,
audio service enabled and running, panel supervision unconditional.

Co-Authored-By: Claude Opus 5 <[email protected]>

tools/build-release.sh

@@ -207,13 +207,19 @@ install -Dm644 \
 # a dock, a window helper, a notification wrapper or a compositor config in
 # the list above.
 
+# Not /etc/xdg: plasma-workspace already owns files by these names there.
+# The classroom's directory goes in front of it in XDG_CONFIG_DIRS instead.
 for setting in kdeglobals kwinrc kwinrulesrc kscreenlockerrc ksmserverrc \
         krunnerrc plasmanotifyrc; do
     install -Dm644 \
         "$ROOT/config/plasma/xdg/${setting}" \
-        "$STAGE/etc/xdg/${setting}"
+        "$STAGE/etc/ekklesia-classroom/xdg/${setting}"
 done
 
+install -Dm644 \
+    "$ROOT/config/environment/90-ekklesia-classroom.conf" \
+    "$STAGE/etc/environment.d/90-ekklesia-classroom.conf"
+
 for scheme in EkklesiaDark EkklesiaLight; do
     install -Dm644 \
         "$ROOT/config/plasma/colors/${scheme}.colors" \